Merge pull request geekan#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"):