Allow aborting notification push

src/notifications.js

@@ -191,6 +191,10 @@ var utils = require('./utils');
 				plugins.fireHook('filter:notification.push', { notification: notification, uids: uids }, next);
 			},
 			function (data, next) {
+				if (!data || !data.notification) {
+					return callback();
+				}
+
 				uids = data.uids;
 				notification = data.notification;