rpc: use null for optional parameters

diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 84ceaaf..889d2fd 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -2526,7 +2526,7 @@ static RPCHelpMan scantxoutset()
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Scan already in progress, use action \"abort\" or \"status\"");
         }
 
-        if (request.params.size() < 2) {
+        if (request.params[1].isNull()) {
             throw JSONRPCError(RPC_MISC_ERROR, "scanobjects argument is required for the start action");
         }
 
diff --git a/src/rpc/output_script.cpp b/src/rpc/output_script.cpp
index 4605f81..19de77e 100644
--- a/src/rpc/output_script.cpp
+++ b/src/rpc/output_script.cpp
@@ -353,7 +353,7 @@ static RPCHelpMan deriveaddresses()
                 throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, error);
             }
             auto& desc = descs.at(0);
-            if (!desc->IsRange() && request.params.size() > 1) {
+            if (!desc->IsRange() && !request.params[1].isNull()) {
                 throw JSONRPCError(RPC_INVALID_PARAMETER, "Range should not be specified for an un-ranged descriptor");
             }
 
diff --git a/src/rpc/server.cpp b/src/rpc/server.cpp
index 34f19df..a519089 100644
--- a/src/rpc/server.cpp
+++ b/src/rpc/server.cpp
@@ -139,7 +139,7 @@ static RPCHelpMan help()
         [&](const RPCHelpMan& self, const JSONRPCRequest& jsonRequest) -> UniValue
 {
     std::string strCommand;
-    if (jsonRequest.params.size() > 0) {
+    if (!jsonRequest.params[0].isNull()) {
         strCommand = jsonRequest.params[0].get_str();
     }
     if (strCommand == "dump_all_command_conversions") {
diff --git a/src/wallet/rpc/elements.cpp b/src/wallet/rpc/elements.cpp
index b455c33..ec57d11 100644
--- a/src/wallet/rpc/elements.cpp
+++ b/src/wallet/rpc/elements.cpp
@@ -325,7 +325,7 @@ RPCHelpMan initpegoutwallet()
 
     // Generate a new key that is added to wallet or set from argument
     CPubKey online_pubkey;
-    if (request.params.size() < 3) {
+    if (request.params[2].isNull()) {
         std::string error;
         if (!pwallet->GetOnlinePakKey(online_pubkey, error)) {
             throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, error);
@@ -342,7 +342,7 @@ RPCHelpMan initpegoutwallet()
 
     // Parse offline counter
     int counter = 0;
-    if (request.params.size() > 1) {
+    if (!request.params[1].isNull()) {
         counter = request.params[1].getInt<int>();
         if (counter < 0 || counter > 1000000000) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "bip32_counter must be between 0 and 1,000,000,000, inclusive.");
@@ -499,7 +499,7 @@ RPCHelpMan sendtomainchain_base()
         throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount for send");
 
     bool subtract_fee = false;
-    if (request.params.size() > 2) {
+    if (!request.params[2].isNull()) {
         subtract_fee = request.params[2].get_bool();
     }
 
@@ -520,7 +520,7 @@ RPCHelpMan sendtomainchain_base()
 
     EnsureWalletIsUnlocked(*pwallet);
 
-    bool verbose = request.params[3].isNull() ? false: request.params[3].get_bool();
+    bool verbose = request.params[3].isNull() ? false : request.params[3].get_bool();
     mapValue_t mapValue;
     CCoinControl no_coin_control; // This is a deprecated API
     return SendMoney(*pwallet, no_coin_control, recipients, std::move(mapValue), verbose, true /* ignore_blind_fail */);
@@ -612,7 +612,7 @@ RPCHelpMan sendtomainchain_pak()
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid amount for send, must send more than 0.00100000 BTC");
 
     bool subtract_fee = false;
-    if (request.params.size() > 2) {
+    if (!request.params[2].isNull()) {
         subtract_fee = request.params[2].get_bool();
     }
 
@@ -825,7 +825,7 @@ static UniValue createrawpegin(const JSONRPCRequest& request, T_tx_ref& txBTCRef
     std::vector<unsigned char> txOutProofData = ParseHex(request.params[1].get_str());
 
     std::set<CScript> claim_scripts;
-    if (request.params.size() > 2) {
+    if (!request.params[2].isNull()) {
         const std::string claim_script = request.params[2].get_str();
         if (!IsHex(claim_script)) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Given claim_script is not hex.");
@@ -1270,12 +1270,12 @@ RPCHelpMan blindrawtransaction()
     }
 
     bool ignore_blind_fail = true;
-    if (request.params.size() > 1) {
+    if (!request.params[1].isNull()) {
         ignore_blind_fail = request.params[1].get_bool();
     }
 
     std::vector<std::vector<unsigned char> > auxiliary_generators;
-    if (request.params.size() > 2) {
+    if (!request.params[2].isNull()) {
         UniValue assetCommitments = request.params[2].get_array();
         if (assetCommitments.size() != 0 && assetCommitments.size() < tx.vin.size()) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Asset commitment array must have at least as many entries as transaction inputs.");
@@ -1543,11 +1543,11 @@ RPCHelpMan issueasset()
         throw JSONRPCError(RPC_TYPE_ERROR, "Issuance must have one non-zero component");
     }
 
-    bool blind_issuances = request.params.size() < 3 || request.params[2].get_bool();
+    bool blind_issuances = request.params[2].isNull() || request.params[2].get_bool();
 
     // Check for optional contract to hash into definition
     uint256 contract_hash;
-    if (request.params.size() >= 4) {
+    if (!request.params[3].isNull()) {
         contract_hash = ParseHashV(request.params[3], "contract_hash");
     }
 
@@ -1739,7 +1739,7 @@ RPCHelpMan listissuances()
 
     std::string assetstr;
     CAsset asset_filter;
-    if (request.params.size() > 0) {
+    if (!request.params[0].isNull()) {
         assetstr = request.params[0].get_str();
         asset_filter = GetAssetFromString(assetstr);
     }
diff --git a/test/functional/feature_fedpeg.py b/test/functional/feature_fedpeg.py
index 646f19d..f30ab5f 100755
--- a/test/functional/feature_fedpeg.py
+++ b/test/functional/feature_fedpeg.py
@@ -601,6 +601,12 @@ class FedPegTest(BitcoinTestFramework):
 
         peg_out_txid = sidechain.sendtomainchain(some_btc_addr, 1)
 
+        self.log.info("sendtomainchain with null argument")
+        verbose_result = sidechain.sendtomainchain(some_btc_addr, 1, None, True)
+        assert isinstance(verbose_result, dict)
+        assert 'txid' in verbose_result
+        assert 'fee_reason' in verbose_result
+
         peg_out_details = sidechain.decoderawtransaction(sidechain.getrawtransaction(peg_out_txid))
         # peg-out, change, fee
         assert len(peg_out_details["vout"]) == 3
diff --git a/test/functional/feature_issuance.py b/test/functional/feature_issuance.py
index 5fe0785..42bbbc8 100755
--- a/test/functional/feature_issuance.py
+++ b/test/functional/feature_issuance.py
@@ -115,12 +115,19 @@ class IssuanceTest(BitcoinTestFramework):
         # Make sure test starts with no initial issuance.
         assert_equal(len(self.nodes[0].listissuances()), 0)
 
+        self.log.info("listissuances with null argument")
+        assert_equal(self.nodes[0].listissuances(None), [])
+
         # Unblinded issuance of asset
         contract_hash = "deadbeef"*8
         issued = self.nodes[0].issueasset(1, 1, False, contract_hash)
         balance = self.nodes[0].getwalletinfo()["balance"]
         assert_equal(balance[issued["asset"]], 1)
         assert_equal(balance[issued["token"]], 1)
+
+        self.log.info("issueasset with null argument")
+        assert_equal(len(self.nodes[0].listissuances(None)), len(self.nodes[0].listissuances()))
+
         # Quick unblinded reissuance check, making 2*COIN total
         self.nodes[0].reissueasset(issued["asset"], 1)
 
diff --git a/test/functional/feature_pak.py b/test/functional/feature_pak.py
index 1ed0338..a3e609e 100755
--- a/test/functional/feature_pak.py
+++ b/test/functional/feature_pak.py
@@ -76,6 +76,11 @@ class PAKTest (BitcoinTestFramework):
         assert_equal(new_init["address_lookahead"][0], init_results[1]["address_lookahead"][2])
         assert new_init["liquid_pak"] != init_results[1]["liquid_pak"]
 
+        self.log.info("initpegoutwallet with null argument")
+        null_pak_init = self.nodes[2].initpegoutwallet(xpub, 5, None)
+        assert_equal(self.nodes[2].getwalletpakinfo()["bip32_counter"], "5")
+        assert null_pak_init["liquid_pak"]
+
         # Restart and connect peers to check wallet persistence
         self.stop_nodes()
         self.start_nodes()
@@ -192,6 +197,12 @@ class PAKTest (BitcoinTestFramework):
         wpkh_stmc = self.nodes[1].sendtomainchain("", 1)
         wpkh_txid = wpkh_stmc['txid']
 
+        self.log.info("sendtomainchain with null argument")
+        verbose_stmc = self.nodes[1].sendtomainchain("", 1, None, True)
+        assert isinstance(verbose_stmc, dict)
+        assert 'txid' in verbose_stmc
+        assert 'fee_reason' in verbose_stmc
+
         # Also check some basic return fields of sendtomainchain with pak
         assert_equal(wpkh_stmc["bitcoin_address"], wpkh_info["address_lookahead"][0])
         validata = self.nodes[1].validateaddress(wpkh_stmc["bitcoin_address"])
diff --git a/test/functional/feature_pegin_subsidy.py b/test/functional/feature_pegin_subsidy.py
index 49e8758..04120d4 100755
--- a/test/functional/feature_pegin_subsidy.py
+++ b/test/functional/feature_pegin_subsidy.py
@@ -340,6 +340,16 @@ class PeginSubsidyTest(BitcoinTestFramework):
         assert_equal(len(pegin_tx["decoded"]["vout"]), 2)
         self.generate(sidechain2, 1, sync_fun=sync_sidechain)
 
+        self.log.info("createrawpegin with null argument")
+        txid, vout, txoutproof, bitcoin_txhex, claim_script = parent_pegin(parent, sidechain2, amount=1.0, feerate=2.0)
+        pegintx = sidechain2.createrawpegin(bitcoin_txhex, txoutproof, None, 2.0)
+        signed = sidechain2.signrawtransactionwithwallet(pegintx["hex"])
+        assert_equal(signed["complete"], True)
+        pegin_txid = sidechain2.sendrawtransaction(signed["hex"])
+        pegin_tx = sidechain2.gettransaction(pegin_txid, True, True)
+        assert_equal(len(pegin_tx["decoded"]["vout"]), 2)
+        self.generate(sidechain2, 1, sync_fun=sync_sidechain)
+
         self.log.info("claimpegin before enforcement, with validatepegin, below threshold")
         txid, vout, txoutproof, bitcoin_txhex, claim_script = parent_pegin(parent, sidechain, amount=1.0, feerate=2.0)
         pegin_txid = sidechain.claimpegin(bitcoin_txhex, txoutproof, claim_script)
diff --git a/test/functional/rpc_deriveaddresses.py b/test/functional/rpc_deriveaddresses.py
index 96b8444..042a644 100755
--- a/test/functional/rpc_deriveaddresses.py
+++ b/test/functional/rpc_deriveaddresses.py
@@ -17,6 +17,7 @@ class DeriveaddressesTest(BitcoinTestFramework):
         descriptor = "wpkh(tprv8ZgxMBicQKsPd7Uf69XL1XwhmjHopUGep8GuEiJDZmbQz6o58LninorQAfcKZWARbtRtfnLcJ5MQ2AtHcQJCCRUcMRvmDUjyEmNUWwx8UbK/1/1/0)#t6wfjs64"
         address = "ert1qjqmxmkpmxt80xz4y3746zgt0q3u3ferrfpgxn5"
         assert_equal(self.nodes[0].deriveaddresses(descriptor), [address])
+        assert_equal(self.nodes[0].deriveaddresses(descriptor, None), [address])
 
         descriptor = descriptor[:-9]
         assert_raises_rpc_error(-5, "Missing checksum", self.nodes[0].deriveaddresses, descriptor)
diff --git a/test/functional/rpc_help.py b/test/functional/rpc_help.py
index db4f895..5143e29 100755
--- a/test/functional/rpc_help.py
+++ b/test/functional/rpc_help.py
@@ -97,6 +97,9 @@ class HelpRpcTest(BitcoinTestFramework):
         # invalid argument
         assert_raises_rpc_error(-3, "JSON value of type number is not of expected type string", node.help, 0)
 
+        # null argument
+        assert_equal(node.help(None), node.help())
+
         # help of unknown command
         assert_equal(node.help('foo'), 'help: unknown command: foo')
 
diff --git a/test/functional/rpc_scantxoutset.py b/test/functional/rpc_scantxoutset.py
index 9c74ea0..428763e 100755
--- a/test/functional/rpc_scantxoutset.py
+++ b/test/functional/rpc_scantxoutset.py
@@ -140,6 +140,7 @@ class ScantxoutsetTest(BitcoinTestFramework):
 
         # Check that second arg is needed for start
         assert_raises_rpc_error(-1, "scanobjects argument is required for the start action", self.nodes[0].scantxoutset, "start")
+        assert_raises_rpc_error(-1, "scanobjects argument is required for the start action", self.nodes[0].scantxoutset, "start", None)
 
         # Check that invalid command give error
         assert_raises_rpc_error(-8, "Invalid action 'invalid_command'", self.nodes[0].scantxoutset, "invalid_command")
diff --git a/test/functional/wallet_fundrawtransaction.py b/test/functional/wallet_fundrawtransaction.py
index 8d6d324..15326fc 100755
--- a/test/functional/wallet_fundrawtransaction.py
+++ b/test/functional/wallet_fundrawtransaction.py
@@ -706,6 +706,9 @@ class RawTransactionsTest(BitcoinTestFramework):
         fundedTx = wallet.fundrawtransaction(rawtx)
         blindedTx = wallet.blindrawtransaction(fundedTx['hex'])
         assert fundedTx["changepos"] != -1
+        self.log.info("blindrawtransaction with null argument")
+        blindedTx_null_commitments = wallet.blindrawtransaction(fundedTx['hex'], None, None, False)
+        assert blindedTx_null_commitments
 
         # Now we need to unlock.
         with WalletUnlock(wallet, "test"):