Fix for exception thrown if no delimiters were specified

[alex-petrov-vt]

Assigned default values for delimiters to fix the exception thrown
Fixes #6624

• Change in CHANGELOG.md described (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.

[koppor]

Thank you for the PR!

Could you also add a CHANGELOG.md entry, too?

[koppor]

I made a suggestion reusing an existing constant.

When this is changed, this is good to go.

We don't have a constant for ,. In case you want to work on the code quality of JabRef, this would be a good thing to start with...

Maybe even make the default delimiter configurable in the preferences.

[koppor]

I commited my update. The required "import" is not added. Maybe, someone can add it. Otherwise, I'll try to find time on the weekend and go ahead with merging; this is some kind of hotfix and should go in early.