PR #457 disallowed adding empty folders in order to compensate for some
limitations with the Magic-Folder "status" API. Those limitations have
since been resolved (specifically, by the addition of  `last-scan` and
`last-poll` fields), making it possible to make determinations about
the sync status of empty folders. Accordingly, this change reverts #457,
allowing empty Magic-Folders to be added again (without those folders
appearing to be stuck in a "Loading..." state).