Add test case for CG operator return type

[mielvds]

Summary of changes

Test case for #652, which seems to indicate that the issue has already been solved

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.94%. remained the same when pulling c5c54a0 on mielvds:fix-652-sum-of-conjunctive-graph-return-type into 247ddb5 on RDFLib:main.

[aucampia]

pre-commit.ci autofix

[aucampia]

Thanks for adding these tests to confirm the issue can be closed.

[mielvds]

cool! Don't forget to close #652

[aucampia]

cool! Don't forget to close #652

Done now, thanks for the reminder.