fix: cannot send message when debug with multiple model with conversa…

[xuzuodong]

…tion opener

Summary

This PR follows up #8627 and #8682, resolves cannot sending first message in multi-model debug panel if Conversation Opener is enabled.

Screenshots

Before:	After:
2024-12-04.15.34.26.mov	2024-12-04.15.33.48.mov

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[cuckootan]

good job!