message: Fixed error that notification with only text couldn't be created.
diff: @@ -87,6 +87,10 @@ export class NotificationController {
  }
  async shouldCreateNotification(user: IUser, changedCourse: ICourse, changedUnit: IUnit = null) {
  + if (!changedCourse && !changedUnit) {
  + // The notificaiton does not depend on any unit/course. We can create a notification.
  + return true;
  + }
  if (!changedUnit) {
  return (await Notification.count({'user': user._id, 'changedCourse': changedCourse._id})) === 0;
  }
  