Skip handling of non-present DOIs

[koppor]

Follow-up to #14357

Somehow, crossref wasn't tested properly.

Also fixes the state handling of RuleBasedPlainCitationParser to allow re-used of the object for each parsing run.

Steps to test

./gradlew :jabkit:run --args="get-cited-works --provider=crossref 10.47397/tb/44-3/tb138kopp-jabref"

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.

[Siedlerchr]

add a test?

[koppor]

add a test?

#Futurework - fetcher is based on the data of the provider. Would take longer to write down different possible returned JSON files. -- Since the fetcher also automatically fetches data from the DOI, it gets even more complicated 😅