cg: get attribute for requested controller in cg_is_empty

This was changed SYSTEMD_CGROUP_CONTROLLER instead of the provided
parameter, which broke sd-cgls. Change it back.

Fixes https://github.com/systemd/systemd/issues/38426

Follow-up for c3f900770d

src/basic/cgroup-util.c

@@ -773,7 +773,7 @@ int cg_is_empty(const char *controller, const char *path) {
         if (empty_or_root(path))
                 return false;
 
-        r = cg_get_keyed_attribute(SYSTEMD_CGROUP_CONTROLLER, path, "cgroup.events", STRV_MAKE("populated"), &t);
+        r = cg_get_keyed_attribute(controller, path, "cgroup.events", STRV_MAKE("populated"), &t);
         if (r == -ENOENT)
                 return true;
         if (r < 0)