FIX: Allow pending posts to have empty titles (#23366)

It's possible for a pending post to have a nil title. The PendingPostSerializer should not throw an error when this is encountered.

app/serializers/pending_post_serializer.rb

@@ -22,6 +22,6 @@ class PendingPostSerializer < ApplicationSerializer
   end
 
   def title
-    payload["title"] || topic.title
+    payload["title"] || topic&.title
   end
 end