Prior, an administrator viewing a project's Labels page would see _all_
labels from every project they had access to, rather than only the
labels of that specific project (if any).
This was not an information disclosure, as admins have access to
everything, but it was a performance issue.
If you attempt to move an issue from one project to another and leave
labels blank, LabelsFinder would assign all labels in the new project
to that issue. The issue is that :title is passed along to the Finder,
but since it appears empty no filtering is done. As a result, all
labels in the group are returned. This fix handles that case.
Closes#23668
