例行检查：

• 我已确认目前没有类似 issue
• 我已确认我已升级到最新版本
• 我已完整查看过项目 README,尤其是常见问题部分
• 我理解并愿意跟进此 issue,协助测试和提供反馈
• 我理解并认可上述内容，并理解项目维护者精力有限，不遵循规则的 issue 可能会被无视或直接关闭

问题描述：
DeepSeek 流式输出最后记录 role = null
官方 api 输出：

{
  "id": "c71316c0-c2ff-4702-8e29-b201dffbf084",
  "choices": [
    {
      "index": 0,
      "delta": {
        "content": "",
        "role": null
      },
      "finish_reason": "stop",
      "logprobs": null
    }
  ],
  "created": 1715574938,
  "model": "deepseek-chat",
  "system_fingerprint": null,
  "object": "chat.completion.chunk",
  "usage": {
    "prompt_tokens": 10,
    "completion_tokens": 37,
    "total_tokens": 47
  }
}

one-api 输出：

{
  "id": "f6b7c79f-0bc5-46b4-8865-572e04d78675",
  "choices": [
    {
      "index": 0,
      "delta": {
        "content": "",
        "role": null
      },
      "finish_reason": "stop",
      "logprobs": null
    }
  ],
  "created": 1715571416,
  "model": "deepseek-chat",
  "system_fingerprint": null,
  "object": "chat.completion.chunk",
  "usage": {
    "prompt_tokens": 10,
    "completion_tokens": 52,
    "total_tokens": 62
  }
}