Fix issue #11938: Tab in empty text area field should focus the next field

[yeonissa]

I made a small fix for issue #11938, where pressing the "Tab" key in an empty text area field should move the focus to the next field. This was achieved by adding an event filter to check for the KeyCode.TAB event and moving the focus when the text area is empty. This functionality ensures better user experience in text navigation.

This PR only modifies the behavior of the tab key in text areas and does not involve other major changes.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.