added test and bugfix for the custom groupconcat separator "" (issue#2473)

[WhiteGobo]

Summary of changes

added test for this case
added bugfix for missing separator in groupconcat
groupconcat checked with generic bool if default " " should be used.Now it checks if aggregator has set separator not None

Also added some annotations

Bugfix for this issue

Fixes #2473

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.904% (+0.001%) from 90.903% when pulling d6ed5f2 on WhiteGobo:bug_groupconcat_uses_separator into 0ea6ca5 on RDFLib:main.

[aucampia]

pre-commit.ci autofix

[aucampia]

Thanks for the fix @WhiteGobo, changes look good.