Return main menu if no sub menu items are found (http://christianvarga.com/how-to-get-submenu-items-from-a-wordpress-menu-based-on-parent-or-sibling/#comment-1710664829)

functions.php

<?php

function my_wp_nav_menu( $theme_location = 'primary', $container = false, $items_wrap = "<div class='widget'><nav><ul>%3\$s</ul></nav></div>" ) {
  $menu = wp_nav_menu(array(
    'container'      => $container,
    'items_wrap'     => $items_wrap,
    'sub_menu'       => true,
    'theme_location' => $theme_location,
    'echo'           => false
    ));

  if ( $menu == '' ) {
    $menu = wp_nav_menu(array(
      'container'      => $container,
      'items_wrap'     => $items_wrap,
      'depth'          => 1,
      'theme_location' => $theme_location,
      'echo'           => false
      ));
  }

  echo $menu;
}