Plan to fix the problem:A particular empty entry can be added without warning message.

[xiaoyanghuo]

This is group14 from the course "Open Source Development".

We are planning to fix #8645, which shows that a particular empty entry can be added without warning message.

• 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.

[ThiloteE]

I edited your introduction commit message to conform to GitHubs syntax of linking pull-requests to issues:

• Before: "We are planning to fix the issue https://github.com/JabRef/jabref/issues/8645" (does not link the pull-request)
• After: "We are planning to fix #8645 (links the pull-request)

[Siedlerchr]

Please do your testing on your own Repos!