fix: Button label issue

[Dhruwang]

What does this PR do?

Fixes https://github.com/formbricks/internal/issues/117

By default we dont have keys for buttonLabel and backButtonLabel, when a user attempts to modify the buttonLabel in a question, a key for it is automatically generated if it doesn't already exist. However, if this key is subsequently set to an empty value, it triggers a validation error. To address this issue, we now remove the key if it is left empty.

How should this be tested?

1. Change next button label or back button label for a question
2. Set it empty
3. Try to save the survey

Checklist

Required

• Filled out the "How to test" section in this PR
• Read How we Code at Formbricks
• Self-reviewed my own code
• Commented on my code in hard-to-understand bits
• Ran pnpm build
• Checked for warnings, there are none
• Removed all console.logs
• Merged the latest changes from main onto my branch with git pull origin main
• My changes don't cause any responsiveness issues
• First PR at Formbricks? Please sign the CLA! Without it we wont be able to merge it 🙏

Appreciated

• If a UI change was made: Added a screen recording or screenshots to this PR
• Updated the Formbricks Docs if changes were necessary

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[github-actions]

Thank you for following the naming conventions for pull request titles! 🙏

[pandeymangg]

LGTM! 🚀