mgr/rbd_support: level_spec is optional for schedule list/status

mgr/rbd_support: level_spec is optional for schedule list/status

Commit fea6fdff4c74 ("mgr/rbd_support: level_spec passed to some
commands is not optional") is wrong.  While it is true that a valid
level_spec is needed to create a LevelSpec instance, an empty string
is very much a valid level spec -- it signifies "all levels".

This wasn't caught because within Ceph these commands are wrapped by
rbd CLI which injects an empty string in get_level_spec_args().

Fixes: https://tracker.ceph.com/issues/54058
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>