Fix crash when sending notification to non-existent user

When renaming a discussion, an attempt is made to send a notification to the discussion's author. However, there is no check to see if the user account still exists - this can lead to a crash. While the check should technically be in the initiating code, it will probably slip through the cracks in other scenarios/extensions, so it's probably best that we safe-guard against this in the NotificationSyncer itself.

src/Core/Notification/NotificationSyncer.php

@@ -85,6 +85,10 @@ class NotificationSyncer
         // it isn't marked as deleted. If they don't, we will want to create a
         // new record for them.
         foreach ($users as $user) {
+            if (! ($user instanceof User)) {
+                continue;
+            }
+
             $existing = $toDelete->first(function ($i, $notification) use ($user) {
                 return $notification->user_id === $user->id;
             });