[Bug] Fix chat completion empty input

[Edenzzzz]

Motivation

Closes #3637 (comment). The server throws a confusing Input should be a valid list if request content is None. This fix instead throws a descriptive error. Subsequent changes are needed on the tool calling end to remove the logic of passing in None, which is illegal.

Modifications

Checklist

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[Edenzzzz]

cc @shuaills Please validate the change, thanks.

[shuaills]

Thanks, I'll have a look.

[shuaills]

Great work, thanks

[shuaills]

LGTM