Fix OpenAiInferenceEngine

[yifanmai]

Fixes two issues as follows:

Sets logprobs to True in the request, because it is required since top_logprobs is set to 20. Otherwise, this error occurs:

  File "/path/to/lib/python3.8/site-packages/openai/_base_client.py", line 1046, in _request
    raise self._make_status_error_from_response(err.response) from None
openai.BadRequestError: Error code: 400 - {'error': {'message': "The 'top_logprobs' parameter is only allowed when 'logprobs' is enabled.", 'type': 'invalid_request_error', 'param': 'top_logprobs', 'code': None}}

Removes request parameters from the OpenAI request if the value is null. Otherwise, this error occurs:

  File "/path/to/lib/python3.8/site-packages/openai/_base_client.py", line 1046, in _request
    raise self._make_status_error_from_response(err.response) from None
openai.BadRequestError: Error code: 400 - {'error': {'message': "Invalid type for 'parallel_tool_calls': expected a boolean, but got null instead.", 'type': 'invalid_request_error', 'param': 'parallel_tool_calls', 'code': 'invalid_type'}}