Fix GMs showing up on refresh after being hidden (#5702)

webapp/stores/notification_store.jsx

@@ -82,6 +82,10 @@ class NotificationStoreClass extends EventEmitter {
                 title = channel.display_name;
             }
 
+            if (title === '') {
+                title = msgProps.channel_display_name;
+            }
+
             let notifyText = post.message.replace(/\n+/g, ' ');
             if (notifyText.length > 50) {
                 notifyText = notifyText.substring(0, 49) + '...';