title: How to order products by meta key and show also products where this meta key is not set
body: <p>I want to order products by specific meta key but I want to show all products including those ones which has not set this meta key. I want to show products with set meta key first and then the rest of the products. 
  Unfortunatelly this soluition woks only to show products whose has set that meta key:      </p>
  
  <pre><code>add_filter('woocommerce_get_catalog_ordering_args', 'custom_woocommerce_get_catalog_ordering_args');
  
  function custom_woocommerce_get_catalog_ordering_args( $args ) {
      global $wp_query;
      if (isset($_GET['orderby'])) {
          switch ($_GET['orderby']) :
              case 'myorder' :
                  $args['order'] = 'DESC';
                  $args['meta_key'] = 'myorder';
                  $args['orderby'] = 'meta_value_num';
              break;
          endswitch;
      }
      return $args;
  }
  </code></pre>