Check for single user case

This fixes an issue when running
extensions/BlueSpiceEchoConnector/maintenance/testNotification.php

PHP Warning: foreach() argument must be of type array|object, string given at extensions/BlueSpiceExpiry/src/Notification/Expiry.php:26

Also prevent empty "comment".

ERM33306

[4.3.x]

Change-Id: I888c71401fc84d7cdb2b18e0e719e54ee3a6c1e0