ceph-volume: fix `simple scan`

ceph-volume: fix `simple scan`

`lsblk_all()` should return an empty dict `{}` if nothing was found.
If we raise `RuntimeError()` then the loop in `scan.Scan.main` will stop
and make ceph-volume fails because we don't try to catch this exception.
`scan.Scan.main()` has its own logic in order to detect the given path
is a ceph-disk created OSD anyway.

Fixes: https://tracker.ceph.com/issues/56482
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>