Self describing.
Removed also artifact from #13867

Feel free to pick some classes and push some commits with null fixes to this PR.

Steps to test

Run gradle :jablib:build, see if null warnings have bin fixed.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [.] I manually tested my changes in running JabRef (always required)
• [.] I added JUnit tests for changes (if applicable)
• [.] I added screenshots in the PR description (if change is visible to the user)
• [.] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [.] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.