fix(types/utils.py): allow passing role for message object

#5621

litellm/types/utils.py

@@ -325,7 +325,7 @@ def __init__(
     ):
         init_values = {
             "content": content,
-            "role": "assistant",
+            "role": role or "assistant",  # handle null input
             "function_call": (
                 FunctionCall(**function_call) if function_call is not None else None
             ),