Fix JS notification error on initialization.

The last notification is initially null resulting in an exception being
thrown on startup.

homeassistant/components/frontend/www_static/polymer/components/ha-notifications.html

@@ -24,10 +24,12 @@
         var toast = this.$.toast;
         var notification = notificationStore.lastNotification;
 
+        if (notification) {
           this.lastId = notification.id;
           toast.text = notification.message;
           toast.show();
         }
+      }
     },
 
   }, storeListenerMixIn));