Refactor processCitation in GrobidService to match processPdf

[btut]

GrobidService.processCitation now returns an Optional to match the signature of processPDF.
This also makes the tests more expressive, as we can compare the BibEntry to an expected BibEntry instead of comparing Strings.

• [] Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• [] Screenshots added in PR description (for UI changes)
• [] Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

At least better than assertTrue. I would prefer if the response deliverd by the processCitaiton would be directly a BibEntry instead of a string, so you could compare two bib entries:

[btut]

At least better than assertTrue. I would prefer if the response deliverd by the processCitaiton would be directly a BibEntry instead of a string, so you could compare two bib entries:

I agree that would make it more consistent with processPDF, so I changed it.

[Siedlerchr]

You have the wrong junit, it. must be import static org.junit.jupiter.api.Assertions.assertTrue;

[Siedlerchr]

Much better now! :)