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

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

litellm/tests/test_completion.py

@@ -68,6 +68,51 @@ def test_completion_custom_provider_model_name():
         pytest.fail(f"Error occurred: {e}")
 
 
+def _openai_mock_response(*args, **kwargs) -> litellm.ModelResponse:
+    _data = {
+        "id": "chatcmpl-123",
+        "object": "chat.completion",
+        "created": 1677652288,
+        "model": "gpt-3.5-turbo-0125",
+        "system_fingerprint": "fp_44709d6fcb",
+        "choices": [
+            {
+                "index": 0,
+                "message": {
+                    "role": None,
+                    "content": "\n\nHello there, how may I assist you today?",
+                },
+                "logprobs": None,
+                "finish_reason": "stop",
+            }
+        ],
+        "usage": {"prompt_tokens": 9, "completion_tokens": 12, "total_tokens": 21},
+    }
+    return litellm.ModelResponse(**_data)
+
+
+def test_null_role_response():
+    """
+    Test if api returns 'null' role, 'assistant' role is still returned
+    """
+    import openai
+
+    openai_client = openai.OpenAI()
+    with patch.object(
+        openai_client.chat.completions, "create", side_effect=_openai_mock_response
+    ) as mock_response:
+        response = litellm.completion(
+            model="gpt-3.5-turbo",
+            messages=[{"role": "user", "content": "Hey! how's it going?"}],
+            client=openai_client,
+        )
+        print(f"response: {response}")
+
+        assert response.id == "chatcmpl-123"
+
+        assert response.choices[0].message.role == "assistant"
+
+
 def test_completion_azure_command_r():
     try:
         litellm.set_verbose = True