fix Gemini token validation issue with LiteLLM (#735)

Fix for #734

Author: handrew

src/agents/extensions/models/litellm_model.py

@@ -111,12 +111,12 @@ async def get_response(
                         input_tokens_details=InputTokensDetails(
                             cached_tokens=getattr(
                                 response_usage.prompt_tokens_details, "cached_tokens", 0
-                            )
+                            ) or 0
                         ),
                         output_tokens_details=OutputTokensDetails(
                             reasoning_tokens=getattr(
                                 response_usage.completion_tokens_details, "reasoning_tokens", 0
-                            )
+                            ) or 0
                         ),
                     )
                     if response.usage