Lo mismo que el post anterior

add_filter( 'get_post_metadata', function( $value, $object_id, $meta_key, $single ) {
	if ( '_thumbnail_id' === $meta_key ) {
		$has_thumbnail = has_post_thumbnail( $object_id );
		if ( ! $has_thumbnail ) {
			$default_image_id = get_option( 'default_featured_image' );
			if ( $default_image_id ) {
				return $single ? $default_image_id : array( $default_image_id );
			}
		}
	}
	return $value;
}, 10, 4 );