Bug: if `hatsune_miku` is an empty general tag, then tagging a post with
`hatsune_miku_(cosplay)` would fail because `hatsune_miku` wasn't a
character tag. This could cause tagging posts to fail when aliasing a
character tag back to an old name that was used in the past, but was now
an empty general tag.
Fix: only treat the *_(cosplay) tag as being in conflict with the base
tag when the base tag is nonempty.