Fix OpenAI server completion_tokens referenced before assignment

[js8544]

When a models return an empty result, the OpenAI server will report an error: local variable completion_tokens referenced before assignment.

[simon-mo]

Good catch. Can you set the completion tokens to 0 in the else clause instead? I believe it is only used in the UsageInfo.

Like UsageInfo(completion_tokens=0).

Thank you!

[js8544]

I don't think using UsageInfo(completion_tokens=0) is correct though. For a non-empty output completion_tokens should be the value set in the if clause.
I do however find out we can use previous_num_tokens[i] instead, to make the code simpler. Please checkout the newest patch.