Set fallback image when no Featured Image is defined

Ref: https://generate.support/topic/setting-default-image-on-dynamic-featured-images/#post-176703

add_filter('generateblocks_dynamic_tag_output', function ($output, $options) {
    $tag_name = $options['tag_name'] ?? '';

    if ('featured_image' === $tag_name) {

        if (empty($output)) {
            return 'https://your-fallback-image-url.com';
        }
    }
    
    return $output;
}, 10, 2);