We should do something more sensible with the default of Exactly One Tag

Summary: Eric stumbled across this: if you define a Property as Exactly One Tag, that all works fine -- but if you fail to enter anything, you get an ugly and problematic empty-name Tag.

Really, this shouldn't be allowed at the Editor level. If a Tag is defined as Exactly One, and there's nothing there, the Editor should highlight that as an error to be fixed.

Right approach might be to change the default value of Tag to something like Missing Tag, to signify that it's an illegal value. The Editor would catch that and display it as empty. It should also put that value back if the value is edited back to empty.