Optimize codec visitor

Author: tconley1428

temporalio/bridge/worker.py

@@ -284,7 +284,8 @@ def __init__(self, f: Callable[[Sequence[Payload]], Awaitable[List[Payload]]]):
 
     async def visit_payload(self, payload: Payload) -> None:
         new_payload = (await self._f([payload]))[0]
-        payload.CopyFrom(new_payload)
+        if new_payload is not payload:
+            payload.CopyFrom(new_payload)
 
     async def visit_payloads(self, payloads: MutableSequence[Payload]) -> None:
         if len(payloads) == 0: