fix: handle empty chunks in response stream

[belingud]

Description

This PR fixes an issue with empty chunks in the response stream that could cause errors when processing streaming responses.

Changes

• Added handling for empty chunk.choices in the response stream
• Skip empty chunks gracefully by yielding an empty string and continuing the loop

Related Issues

• Fixed the streaming API that may return an empty data block causing an out-of-index error.

Additional Notes

This change makes the streaming handler more robust by properly handling edge cases in the response stream without breaking the processing flow.