Add menu_link_content_list cache key for menu if empty

This ensures that the cache is cleared when the first item is added
to the menu.

src/View/MenuViewBuilder.php

@@ -73,6 +73,7 @@ class MenuViewBuilder {
         '[level]=' . $level,
         '[depth]=' . $depth,
         '[expand]=' . (int) $expand,
+        'menu_link_content_list:' . $menu_name
       ];
     }