WordPress - Default menu fallback_cb for non-existent menus

wp_nav_menu_args.php

function hf_default_menu_fallback($nav_elem, $nav_class) {
  echo (!empty($nav_elem) ? '<' . $nav_elem . (!empty($nav_class) ? ' class="' . $nav_class . '">' : '>') : '')
    . '<ul class="menu">'
      . '<li class="menu-item">'
        . '<a href="' . admin_url('nav-menus.php?action=edit&menu=0') . '">Add Menu</a>'
      . '</li>'
    . '</ul>'
  . ($nav_elem !== false ? '</' . $nav_elem . '>' : '');
}

function hf_default_wp_nav_menu_args($args) {
  if(has_nav_menu($args['theme_location']) === false) {
    $args['fallback_cb'] = hf_default_menu_fallback($args['container'], $args['container_class']);
  }
  return $args;
}
add_filter('wp_nav_menu_args', 'hf_default_wp_nav_menu_args');