🐛 fix bug: QueueHelper::storeEmail(): $email null given

Civi/Mailinglistsync/QueueHelper.php

@@ -106,12 +106,14 @@ class QueueHelper {
    * Stores an email address in the queue helper singleton.
    *
    * @param \CRM_Queue_TaskContext $context
-   * @param string $email
+   * @param string|null $email
    *
    * @return bool
    */
-  public static function storeEmail(\CRM_Queue_TaskContext $context, string $email): bool {
-    self::getInstance()->addToEmails($email);
+  public static function storeEmail(\CRM_Queue_TaskContext $context, ?string $email): bool {
+    if (!empty($email)) {
+      self::getInstance()->addToEmails($email);
+    }
     return TRUE;
   }