Setting fallback (default) image to featured image block

I don’t know if you’re still looking for a solution, but you can use the post_thumbnail_html filter in you php file to return a fallback; like so.

/**
 * Set the default image if none exists.
 *
 * @param string $html              The post thumbnail HTML.
 * @param int    $post_id           The post ID.
 * @param int    $post_thumbnail_id The post thumbnail ID.
 * @return html
 */
public function wp_893874_fallback_post_thumbnail_html( $html, $post_id, $post_thumbnail_id ) {
    if ( empty( $html ) ) {
        $html="<img src="http://lorempixel.com/g/400/200" width="400" height="200" loading="lazy" alt="" . get_the_title( $post_id ) . '" />';
    }

    return $html;
}
add_filter( 'post_thumbnail_html', 'wp_893874_fallback_post_thumbnail_html', 5, 3 );