Events in the JcrListener can be cleared when the UserSession is null

The second stacktrace in CMS7-5790 revealed a case where the UserSession can be null when the event listener is called. This can happen when wicket has serialized the user session but the backing JCR Session was not yet logged out. (For the logic of the latter refer to JcrSessionReference.cleanup()). We can clear out the events in the jcr listener in this case because they will not be used anymore. When wicket revives and deserializes the user session again a new JCR session will be created.