fix: Handle empty choices in OpenAI response

[yangbobo2021]

This PR addresses the issue of handling empty choices in OpenAI responses. The changes include:

• Adding a check for non-empty choices array in the response
• Moving processing logic inside the non-empty check
• Improving error handling for malformed responses

These updates will enhance the robustness of our OpenAI integration and prevent potential errors when dealing with unexpected response formats.