Fix NPE when importing old JabRef preferences

[pulkitbajaj]

Closes #14497

Steps to test

1. Download the JabRef.xml file provided in issue Import old jabref preferences fails with npe odel.metadata.UserHostInfo.getUserHostString #14497.
2. Created a temporary JUnit test to reproduce the crash by calling new JabRefCliPreferences().getInternalPreferences().
3. Verified that the test failed with a NullPointerException before the fix.
4. Verified that the test passed successfully after applying the fix.

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.

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AI Disclosure: I used an AI assistant to help analyze the stack trace, understand the root cause (NullPointerException), and draft the reproduction test case. I manually verified the fix and the test results.

[github-actions]

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[jabref-machine]

Hey, we noticed that you force-pushed your changes. Force pushing is a bad practice when working together on a project (mainly because it is not supported well by GitHub itself). Commits are lost and comments on commits lose their context, thus making it harder to review changes.

When the pull request is getting integrated into main, all commits will be squashed anyway. Thus, your individual commit history will not be visible in main.

In future, please avoid that. For now, you can continue working.