fix(test): if there is no alert_id use null as default value

includes/models/class-test.php

@@ -592,10 +592,10 @@ public static function get_all_service_test_ids() {
 	/**
 	 * Set alert for a test.
 	 *
-	 * @param int $post_id The id of the post.
-	 * @param int $alert_id The id of the alert.
+	 * @param int      $post_id The id of the post.
+	 * @param null|int $alert_id The id of the alert.
 	 */
-	public static function set_alert( $post_id = 0, $alert_id = 0 ) {
+	public static function set_alert( $post_id = 0, $alert_id = null ) {
 		global $wpdb;
 
 		$tests_table = Tests_Table::get_table_name();