chore: change LlmRequest.config's default value to be types.GenerateContentConfig() instead of None

PiperOrigin-RevId: 789792582

tests/unittests/models/test_google_llm.py

@@ -1505,7 +1505,6 @@ async def test_computer_use_with_no_config():
       contents=[
           types.Content(role="user", parts=[types.Part.from_text(text="Hello")])
       ],
-      config=None,
   )
 
   # Should not raise an exception