fix(utils.py): handle api assistant returning 'null' role

Fixes https://github.com/BerriAI/litellm/issues/3621

litellm/main.py

@@ -9,6 +9,7 @@
 
 import os, openai, sys, json, inspect, uuid, datetime, threading
 from typing import Any, Literal, Union, BinaryIO
+from typing_extensions import overload
 from functools import partial
 import dotenv, traceback, random, asyncio, time, contextvars
 from copy import deepcopy