6e1fe22 introduced the possiblity for category_users to have a NULL notification_level, so that we can store `last_seen_at` dates without locking the notification level. At the time, this did not affect the topic-tracking-state query. However, the query changes in f434de2 introduced a slight change in behavior.
Previously, a subquery would look for a category_user with notification_level=mute. f434de2 refactored this to remove the subquery, and inverted some of the logic to suit.
The new query checked for `notification_level <> :muted`. If `notification_level` is NULL, this comparison will return NULL. In this scenario, notification_level=NULL means that we should fall back to the default tracking level (regular), and so we want the expression to resolve as true, not false. There was already a check for the existence of the category_users row, but it did not check for the existence of a NOT NULL notification_level.
This commit amends the expression so that the notification_level will only be compared if it is non-null.
5df815c2ee