Fix startup in case of files

[koppor]

Follow-up to #13012

fileUpdateMonitor was not initalized properly.

Additionally, this PR enables assert statements to be evaluted.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] 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.

[trag-bot]

Using assertions for null checks is not recommended for production code as they can be disabled. Consider using JSpecify @nonnull annotations for better null safety.

[trag-bot]

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

[koppor]

Failing build is due to timeout on cache-restore and issues connecting to gradle server.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.