Merge pull request #1253 from aa875982361/fix/none-usage

fix: failure when usage is none

metagpt/provider/openai_api.py

@@ -100,7 +100,7 @@ async def _achat_completion_stream(self, messages: list[dict], timeout=USE_CONFI
  log_llm_stream(chunk_message)
  collected_messages.append(chunk_message)
  if finish_reason:
- if hasattr(chunk, "usage"):
+ if hasattr(chunk, "usage") and chunk.usage is not None:
  # Some services have usage as an attribute of the chunk, such as Fireworks
  usage = CompletionUsage(**chunk.usage)
  elif hasattr(chunk.choices[0], "usage"):