AddTagsPane#add() has to try adding a tag and if the add tag window disappears then it is considered a success, otherwise it is a failure (added existing tag or empty tag).
AddTagsPane#add() waits until the add tag form disappears or the field becomes enabled, the problem is that AFAICT it is never disabled. If the user attempts to add an empty tag, nothing happens at all.
This should IMO be fixed by showing an error notification if the user attempts to save an empty tag, then the error notification can be waited for.
Since that introduces changes to the actual code which carry the possibility of regressions, I think the next best thing is to change the AddTagsPane#add() to wait for an error notification or success and remove addEmptyTagTest since it is trivial and difficult to support right now.