fix(file): serialize explicit empty key list in FileUpdateTransaction

Signed-off-by: anchit-goel <anchitgoel5@gmail.com>

Author: anchit-goel

src/hiero_sdk_python/file/file_update_transaction.py

@@ -164,7 +164,7 @@ def _build_proto_body(self):
         """
         return FileUpdateTransactionBody(
             fileID=self.file_id._to_proto() if self.file_id is not None else None,
-            keys=(KeyListProto(keys=[key._to_proto() for key in self.keys]) if self.keys else None),
+            keys=(KeyListProto(keys=[key._to_proto() for key in self.keys]) if self.keys is not None else None),
             contents=self.contents if self.contents is not None else b"",
             expirationTime=(self.expiration_time._to_protobuf() if self.expiration_time else None),
             memo=(StringValue(value=self.file_memo) if self.file_memo is not None else None),

tests/unit/file_update_transaction_test.py

@@ -202,6 +202,21 @@ def test_build_transaction_body_with_optional_fields(mock_account_ids, file_id):
     assert not transaction_body.fileUpdate.HasField("memo")
 
 
+def test_build_transaction_body_with_empty_keys(mock_account_ids, file_id):
+    """Test building transaction body with an explicit empty keys list sets empty KeyList proto."""
+    operator_id, _, node_account_id, _, _ = mock_account_ids
+
+    file_tx = FileUpdateTransaction(file_id=file_id)
+    file_tx.operator_account_id = operator_id
+    file_tx.set_node_account_ids([node_account_id])
+    file_tx.set_keys([])
+
+    transaction_body = file_tx.build_transaction_body()
+
+    assert transaction_body.fileUpdate.HasField("keys")
+    assert len(transaction_body.fileUpdate.keys.keys) == 0
+
+
 def test_build_scheduled_body(mock_account_ids, file_id):
     """Test building a schedulable file update transaction body."""
     operator_id, _, node_account_id, _, _ = mock_account_ids