repair: Don't crash if no remotes are configured

If no remotes are configured, ostree_repo_remote_list returns NULL
so don't dereference it.

Fixes: https://github.com/flatpak/flatpak/issues/3436

common/flatpak-dir.c

@@ -15140,7 +15140,7 @@ flatpak_dir_delete_mirror_refs (FlatpakDir    *self,
    * OS updates which Flatpak shouldn't touch.
    */
   remotes = ostree_repo_remote_list (repo, NULL);
-  for (i = 0; remotes[i] != NULL; i++)
+  for (i = 0; remotes != NULL && remotes[i] != NULL; i++)
     {
       g_autofree char *remote_collection_id = NULL;