Fix signer_nonce serialization in ContractFunctionResult (hiero-ledger#2293)

Signed-off-by: utkarsh patrikar <utkarshpatrikar@gmail.com>

Author: utkarsh232005

src/hiero_sdk_python/contract/contract_function_result.py

@@ -507,6 +507,6 @@ def _to_proto(self) -> contract_types_pb2.ContractFunctionResult:
             gas=self.gas_available,
             amount=self.amount,
             functionParameters=self.function_parameters,
-            signer_nonce=Int64Value(value=self.signer_nonce),
+            signer_nonce=Int64Value(value=self.signer_nonce) if self.signer_nonce is not None else None,
             contract_nonces=[contract_nonce._to_proto() for contract_nonce in self.contract_nonces or []],
         )

tests/unit/contract_function_result_test.py

@@ -405,3 +405,17 @@ def test_to_proto(contract_function_result):
     assert proto.amount == contract_function_result.amount
     assert proto.functionParameters == contract_function_result.function_parameters
     assert proto.signer_nonce.value == contract_function_result.signer_nonce
+
+
+def test_to_proto_signer_nonce():
+    """Test _to_proto method with None and 0 signer_nonce."""
+    # Test with None
+    result_none = ContractFunctionResult(signer_nonce=None)
+    proto_none = result_none._to_proto()
+    assert not proto_none.HasField("signer_nonce")
+
+    # Test with 0
+    result_zero = ContractFunctionResult(signer_nonce=0)
+    proto_zero = result_zero._to_proto()
+    assert proto_zero.HasField("signer_nonce")
+    assert proto_zero.signer_nonce.value == 0