In the Maniphest task creation form, there is a 'Create New Project' link right below the Projects field. We made a project with this and it assigned an empty hashtag to it.

Looking at the project history, it got created with a blank name and then renamed to what we wanted it to be, which seems like a good reason for an empty hashtag to appear.

Can't repro here because I can't create projects (I assume I lack perms), but can do it at will on my instance.