Remove the last empty assistant message for ChatGPT API call

[bhy]

Due to the way the regex was written, currently there is always an empty message produced at the end of messages list, eg: [{'role': 'system', 'content': 'You are a helpful assistant.'}, {'role': 'user', 'content': 'Whats is the meaning of life??'}, {'role': 'assistant', 'content': ''}].

This PR fix the regex and thus remove this empty message.