manager: Freeze/Thaw: Don't fail units w/o cgroup

Previously, it was impossible to freeze or thaw a slice if it is an
ancestor to a unit that had no running cgroup (i.e. a service with
RemainAfterExit=yes). Instead of failing with EBUSY (which would
confusingly get reported as "Unit has pending job") we just silently
no-op.

I noticed this because we now have user-runtime-dir@.service, which
would make it impossible to freeze/thaw user.slice or user-<UID>.slice

src/core/cgroup.c

@@ -5128,7 +5128,7 @@ int unit_cgroup_freezer_action(Unit *u, FreezerAction action) {
 
         CGroupRuntime *crt = unit_get_cgroup_runtime(u);
         if (!crt || !crt->cgroup_realized)
-                return -EBUSY;
+                return 0; /* No cgroup = nothing running to freeze */
 
         unit_next_freezer_state(u, action, &next, &target);