[bugfix]Fix chunk.choices[0].delta.content is None in query response

[KylinMountain]

Description

When querying, sometimes it will report TypeError: can only concatenate str (not "NoneType") to str, it is because the ChoiceDelta.content is None as following debug log.

ChatCompletionChunk(id='chatcmpl-58de5f44-f801-48c8-9cae-8629e4e5051b', choices=[Choice(delta=ChoiceDelta(content=None, function_call=None, role=None, tool_calls=None), finish_reason='stop', index=0, logprobs=None)], created=1720755079, model='gemma2-9b-it', object='chat.completion.chunk', service_tier=None, system_fingerprint='fp_10c08bf97d', usage=None, x_groq={'id': 'req_01j2jfxhv1ff9s6y2hqhnr0zk1', 'usage': {'queue_time': 0.023216143, 'prompt_tokens': 924, 'prompt_time': 0.032787483, 'completion_tokens': 99, 'completion_time': 0.198, 'total_tokens': 1023, 'total_time': 0.23078748300000002}})

Exception in reduce_response
Traceback (most recent call last):
  File "/Users/evilkylin/Projects/graphrag/graphrag/query/structured_search/global_search/search.py", line 324, in _reduce_response
    search_response = await self.llm.agenerate(
  File "/Users/evilkylin/Projects/graphrag/graphrag/query/llm/oai/chat_openai.py", line 110, in agenerate
    async for attempt in retryer:
  File "/Users/evilkylin/Library/Caches/pypoetry/virtualenvs/graphrag-g0mKwYYC-py3.10/lib/python3.10/site-packages/tenacity/asyncio/__init__.py", line 166, in __anext__
    do = await self.iter(retry_state=self._retry_state)
  File "/Users/evilkylin/Library/Caches/pypoetry/virtualenvs/graphrag-g0mKwYYC-py3.10/lib/python3.10/site-packages/tenacity/asyncio/__init__.py", line 153, in iter
    result = await action(retry_state)
  File "/Users/evilkylin/Library/Caches/pypoetry/virtualenvs/graphrag-g0mKwYYC-py3.10/lib/python3.10/site-packages/tenacity/_utils.py", line 99, in inner
    return call(*args, **kwargs)
  File "/Users/evilkylin/Library/Caches/pypoetry/virtualenvs/graphrag-g0mKwYYC-py3.10/lib/python3.10/site-packages/tenacity/__init__.py", line 398, in <lambda>
    self._add_action_func(lambda rs: rs.outcome.result())
  File "/Users/evilkylin/Projects/miniforge3/lib/python3.10/concurrent/futures/_base.py", line 451, in result
    return self.__get_result()
  File "/Users/evilkylin/Projects/miniforge3/lib/python3.10/concurrent/futures/_base.py", line 403, in __get_result
    raise self._exception
  File "/Users/evilkylin/Projects/graphrag/graphrag/query/llm/oai/chat_openai.py", line 112, in agenerate
    return await self._agenerate(
  File "/Users/evilkylin/Projects/graphrag/graphrag/query/llm/oai/chat_openai.py", line 192, in _agenerate
    full_response += delta
TypeError: can only concatenate str (not "NoneType") to str

Related Issues

Proposed Changes

Since the ChoiceDelta.content is defined as content: Optional[str] = None, we should avoid assign None to delta.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

[Add any additional notes or context that may be helpful for the reviewer(s).]