How to order products by meta key and show also products where this meta key is not set

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:

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;
}