use post slug as default subtitle

add_filter( 'jnews_single_subtitle', 'add_default_subtitle', 99, 2 );

/*
Use Post Slug as default subtitle if the subtitile is empoty.
*/
function add_default_subtitle( $subtitle, $post_id ) {

    if ( empty( $subtitle ) ) {
        $subtitle = ucwords( str_replace( '-', ' ', get_post_field( 'post_name', $post_id ) ) );
    }
    return $subtitle;
}