templater: fix string representation of wrapped None flatten() and stringify() skip None, which means wrappedvalue(None).show() must return '' instead of 'None'. This isn't a problem right now, but we'll encounter it once we start using wrapped types extensively.

templater: show slightly better hint on map operation error It was super hard to find where the templater went wrong if a keyword was evaluated to '' or None.

templater: inline unwraphybrid() flatten() is the solo user of this function.