systemctl: fail in the case that no unit files were found

Previously systemctl died with message

-bash-4.2# systemctl --root /rawhi list-unit-files
(src/systemctl/systemctl.c:868) Out of memory.

in the case that no unit files were found in the --root
or the directory did not exist.

So lets return ENOENT in the case that --root does not exist
and empty list in the case that there are no unit files.

src/shared/install.c

@@ -2044,6 +2044,12 @@ int unit_file_get_list(
         if (root_dir && scope != UNIT_FILE_SYSTEM)
                 return -EINVAL;
 
+        if (root_dir) {
+                r = access(root_dir, F_OK);
+                if (r < 0)
+                        return -errno;
+        }
+
         r = lookup_paths_init_from_scope(&paths, scope, root_dir);
         if (r < 0)
                 return r;