Fix: set up front-end validation in feedback modal

[Daetha]

Closes #1137

Changes

Implemented simple logic: if all the fields are empty, don't send the POST request.

Notes for Reviewer

submit en empty feedback -> check container logs

Checklist

Branching Strategy:

If this is a hotfix: The branch was created from main and will be merged into main.
If this is a feature or documentation change: The branch was created from dev and will be merged into dev.
If this is a feature or documentation branch, I have pulled the latest changes from main into my branch.

Code Quality

• My code follows the style guide.
• I have self-reviewed my code.
• I added comments for hard-to-understand code.
• My changes do not create new warnings in the browser console.

Documentation

• If applicable, PHP code is documented using PHPDoc.
• If applicable, JavaScript code is documented using JSDoc.
• If needed, the ELMO Guide ('./doc/help.html') has been updated.
• If needed, the README has been updated.
• If needed, the API documentation ('./api/v2/docs') has been updated.
• If a new feature was added or a bug fixed, the changelog ('./doc/changelog.html') has been updated.

Testing

• If needed, Playwright tests have been updated or added.
• If needed, unit tests have been updated or added.
• If needed, jest tests have been updated or added.

Known Issues

[What lower priority problems remain?]