What is the difference between a topic with NULL
category_id and a topic with
category_id = (the value for
uncategorized_category_id in the SITE_SETTINGS table)?
If all uncategorized topics are assigned to the Uncategorized category, then possible the column could be made
NOT NULL in a migration? There are still a few
where topics.category_id IS NULL checks in the code which could then be retired for a tiny performance boost.