finish_reason is "" instead of null

[allenhaozi]

When using go sdk requests /completions, finish_reason is "" instead of null

request vllm directly

...
data: {"id":"cmpl-096176162ed84f0e85e6b5aece29f27b","object":"chat.completion.chunk","created":8209815,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"index":0,"delta":{"content":""},"finish_reason":null}]}

data: {"id":"cmpl-096176162ed84f0e85e6b5aece29f27b","object":"chat.completion.chunk","created":8209815,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"index":0,"delta":{"content":"\n"},"finish_reason":null}]}

data: {"id":"cmpl-096176162ed84f0e85e6b5aece29f27b","object":"chat.completion.chunk","created":8209815,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"index":0,"delta":{"content":""},"finish_reason":"stop"}],"usage":{"prompt_tokens":12,"total_tokens":55,"completion_tokens":43}}

Request vllm through go SDK

...
{"id":"cmpl-840b320554774957a66308b67a34ffd2","object":"","created":8210399,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"text":".","index":0,"finish_reason":"","logprobs":{"tokens":null,"token_logprobs":null,"top_logprobs":null,"text_offset":null}}],"usage":{"prompt_tokens":0,"completion_tokens":0,"total_tokens":0}}
{"id":"cmpl-840b320554774957a66308b67a34ffd2","object":"","created":8210399,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"text":"","index":0,"finish_reason":"","logprobs":{"tokens":null,"token_logprobs":null,"top_logprobs":null,"text_offset":null}}],"usage":{"prompt_tokens":0,"completion_tokens":0,"total_tokens":0}}
{"id":"cmpl-840b320554774957a66308b67a34ffd2","object":"","created":8210399,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"text":"\n","index":0,"finish_reason":"","logprobs":{"tokens":null,"token_logprobs":null,"top_logprobs":null,"text_offset":null}}],"usage":{"prompt_tokens":0,"completion_tokens":0,"total_tokens":0}}
{"id":"cmpl-840b320554774957a66308b67a34ffd2","object":"","created":8210399,"model":"public/qwen1-5-72b-chat-int4@main","choices":[{"text":"","index":0,"finish_reason":"stop","logprobs":{"tokens":null,"token_logprobs":null,"top_logprobs":null,"text_offset":null}}],"usage":{"prompt_tokens":8,"completion_tokens":20,"total_tokens":28}}