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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.