util-lib: introduce new empty_or_root() helper (#8746)

We check the same condition at various places. Let's add a trivial,
common helper for this, and use it everywhere.

It's not going to make things much faster or much shorter, but I think a
lot more readable

src/basic/user-util.c

@@ -221,8 +221,7 @@ int get_user_creds_clean(
             (isempty(*shell) || is_nologin_shell(*shell)))
                 *shell = NULL;
 
-        if (home &&
-            (isempty(*home) || path_equal(*home, "/")))
+        if (home && empty_or_root(*home))
                 *home = NULL;
 
         return 0;