FIX: Unable to move pm to public topic (#24903)

* FIX: guard against empty category_ids when creating small action post for changing of category

Co-authored-by: Kelvin Tan <kelv@discourse.org>

lib/post_revisor.rb

@@ -154,7 +154,9 @@ class PostRevisor
   end
 
   def self.create_small_action_for_category_change(topic:, user:, old_category:, new_category:)
-    return if !SiteSetting.create_post_for_category_and_tag_changes
+    if !old_category || !new_category || !SiteSetting.create_post_for_category_and_tag_changes
+      return
+    end
 
     topic.add_moderator_post(
       user,