logind: stop managing VT switches if no sessions are registered on that VT

logind: stop managing VT switches if no sessions are registered on that VT

When no sessions are registered on a given VT (anymore), we should always let
the kernel processes VT switching (instead of simply emitting a warning)
otherwise the requests sent by the kernel are simply ignored making the VT
switch requested by users simply impossible.

Even if it shouldn't happen, this case was encountered in issue #9754, so
better to be safe than sorry.