FIX: TopicQuery for NULL category.topic_id (#20664)

Our schema allows `category.topic_id` to be NULL. Null values shouldn't actually happen in production, but it is very common in tests because `Fabricate(:category)` skips creating the definition topic to improve performance. Before this commit, a NULL category.topic_id would cause all subcategory topics to be excluded from a TopicQuery result. This is because, in postgres, `NULL <> anything` is falsy. Instead, we can use `IS DISTINCT FROM`, which will return true when NULL is compared to a non-NULL value.

lib/topic_query.rb

@@ -692,7 +692,10 @@ class TopicQuery
         result = result.where("topics.category_id IN (?)", Category.subcategory_ids(category_id))
         if !SiteSetting.show_category_definitions_in_topic_lists
           result =
-            result.where("categories.topic_id <> topics.id OR topics.category_id = ?", category_id)
+            result.where(
+              "categories.topic_id IS DISTINCT FROM topics.id OR topics.category_id = ?",
+              category_id,
+            )
         end
       end
       result = result.references(:categories)