Improve ConsistencyCheck code (and one minor other code place)

[koppor]

Follow-up to #13584.

Fixes consistency check to handle unknown entry types again.

(Also to #12433)

Minor code refactorings.

Steps to test

Run consistency check

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.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨