New tests for NumericFieldComparator class

[jvsdurso]

Implementation of Multiple Condition Coverage (MCC) for isNumber method in NumericFieldComparator class. There is no issue for it.

• [X] Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• [X] Tests created for changes (if applicable)
• [X] Manually tested changed features in running JabRef (always required)
• [X] 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.