FIX: prevents exception if thread is not found (#21099)

It is yet to investigate the exact reasons leading to this, but probably due to some delete operation or migration, it seems possible to have a message with a thread_id leading to a non existing thread row. This is only a temporary solution to prevent the crash. We should also probably be more defensive here and not include any of this if threading is not enabled.

plugins/chat/app/serializers/chat/message_serializer.rb

@@ -158,7 +158,7 @@ module Chat
     end
 
     def thread_reply_count
-      object.thread.replies_count
+      object.thread&.replies_count || 0
     end
   end
 end