Fix ID field to be optional

[TimMisiak]

When testing tool calls in streaming mode I noticed that I was getting entirely empty deltas. Dumping the json before trying to deserialize showed that the "id" field is missing after the first message, but the json annotation has the field as required. Here's an example json:

{
    "id": "chatcmpl-...",
    "object": "chat.completion.chunk",
    "created": 1733436687,
    "model": "gpt-4o-2024-08-06",
    "system_fingerprint": "fp_...",
    "choices": [
        {
            "index": 0,
            "delta": {
                "tool_calls": [
                    {
                        "index": 0,
                        "function": {
                            "arguments": "{\""
                        }
                    }
                ]
            },
            "logprobs": null,
            "finish_reason": null
        }
    ]
}

Adding "omitempty" seems to let the json parse correctly and shouldn't break existing users. I tested these changes in a local copy of the lib and it was able to parse successfully.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (774fc9d) to head (b631ade).
Report is 76 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
+ Coverage   98.46%   98.69%   +0.23%     
==========================================
  Files          24       26       +2     
  Lines        1364     1761     +397     
==========================================
+ Hits         1343     1738     +395     
- Misses         15       16       +1     
- Partials        6        7       +1     

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[sashabaranov]

Thank you!