DEV: Prefer nested queries (#23464)

Some sites have a large number of categories and fetching the category
IDs or category topic IDs just to build another query can take a long
time or resources (i.e. memory).

app/controllers/application_controller.rb

@@ -931,7 +931,7 @@ class ApplicationController < ActionController::Base
         Discourse
           .cache
           .fetch(key, expires_in: 10.minutes) do
-            category_topic_ids = Category.pluck(:topic_id).compact
+            category_topic_ids = Category.select(:topic_id).where.not(topic_id: nil)
             @top_viewed =
               TopicQuery
                 .new(nil, except_topic_ids: category_topic_ids)