Merge bitcoin/bitcoin#36012: psbt: Remove unused `IsNull()` methods

### src/psbt.cpp
@@ -31,11 +31,6 @@ PartiallySignedTransaction::PartiallySignedTransaction(const CMutableTransaction
     }
 }
 
-bool PartiallySignedTransaction::IsNull() const
-{
-    return inputs.empty() && outputs.empty() && unknown.empty();
-}
-
 bool PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt)
 {
     // Prohibited to merge two PSBTs over different transactions
@@ -289,11 +284,6 @@ COutPoint PSBTInput::GetOutPoint() const
     return COutPoint(prev_txid, prev_out);
 }
 
-bool PSBTInput::IsNull() const
-{
-    return !non_witness_utxo && witness_utxo.IsNull() && partial_sigs.empty() && unknown.empty() && hd_keypaths.empty() && redeem_script.empty() && witness_script.empty();
-}
-
 void PSBTInput::FillSignatureData(SignatureData& sigdata) const
 {
     if (!final_script_sig.empty()) {
@@ -528,11 +518,6 @@ void PSBTOutput::FromSignatureData(const SignatureData& sigdata)
     m_musig2_participants.insert(sigdata.musig2_pubkeys.begin(), sigdata.musig2_pubkeys.end());
 }
 
-bool PSBTOutput::IsNull() const
-{
-    return redeem_script.empty() && witness_script.empty() && hd_keypaths.empty() && unknown.empty();
-}
-
 bool PSBTOutput::Merge(const PSBTOutput& output)
 {
     hd_keypaths.insert(output.hd_keypaths.begin(), output.hd_keypaths.end());

### src/psbt.h
@@ -321,7 +321,6 @@ class PSBTInput
     std::set<PSBTProprietary> m_proprietary;
     std::optional<int> sighash_type;
 
-    bool IsNull() const;
     void FillSignatureData(SignatureData& sigdata) const;
     void FromSignatureData(const SignatureData& sigdata);
     [[nodiscard]] bool Merge(const PSBTInput& input);
@@ -956,7 +955,6 @@ class PSBTOutput
     CAmount amount;
     CScript script;
 
-    bool IsNull() const;
     void FillSignatureData(SignatureData& sigdata) const;
     void FromSignatureData(const SignatureData& sigdata);
     [[nodiscard]] bool Merge(const PSBTOutput& output);
@@ -1254,7 +1252,6 @@ class PartiallySignedTransaction
     uint32_t tx_version;
     std::optional<uint32_t> fallback_locktime;
 
-    bool IsNull() const;
     uint32_t GetVersion() const;
 
     /** Merge psbt into this. The two psbts must have the same underlying CTransaction (i.e. the

### src/test/fuzz/psbt.cpp
@@ -54,12 +54,10 @@ FUZZ_TARGET(psbt)
         (void)PSBTRoleName(input_analysis.next);
     }
 
-    (void)psbt.IsNull();
     (void)psbt.GetUnsignedTx();
 
     for (const PSBTInput& input : psbt.inputs) {
         (void)PSBTInputSigned(input);
-        (void)input.IsNull();
         PSBTInput input_mod = input;
         CTxOut tx_out;
         if (input.GetUTXO(tx_out)) {
@@ -96,7 +94,6 @@ FUZZ_TARGET(psbt)
     (void)CountPSBTUnsignedInputs(psbt);
 
     for (const PSBTOutput& output : psbt.outputs) {
-        (void)output.IsNull();
         PSBTOutput output_mod = output;
         // A PSBT output must roundtrip to signature data.
         PSBTOutput output_fill{psbt_version, output_mod.amount, output_mod.script};