Fix chat completion create response

[lanvada]

1. Change the Usage to a nullable type.
2. When stream is true, the Delta field in the last object is {}.

{
    "id": "chatcmpl-7vKPd1Z1lthmAqos39G2iWlAtkcKb",
    "object": "chat.completion.chunk",
    "created": 1693897517,
    "model": "gpt-4-0613",
    "choices": [
        {
            "index": 0,
            "delta": {},
            "finish_reason": "stop"
        }
    ]
}

3. Except for the first object, the Roles in other Message objects are empty.

{
    "id": "chatcmpl-7vKPd1Z1lthmAqos39G2iWlAtkcKb",
    "object": "chat.completion.chunk",
    "created": 1693897517,
    "model": "gpt-4-0613",
    "choices": [
        {
            "index": 0,
            "delta": {
                "content": "The"
            },
            "finish_reason": null
        }
    ]
}

4. I saw that you judged whether the Choice is null when processing the result, I think it can be consistent and change the Choice field to be nullable.
5. Except for the last object, the FinishReason in ChatChoiceRespons is null, so change the FinishReason field in ChatChoiceResponse to be nullable