Ensure uniqueness of toplevel category name
As NULL value is always different for the UNIQUE constraint, we must use an EXCLUDE constraint with a COALESCE for when the parent is NULL.
This instance will be upgraded to Heptapod 1.5.0rc (GitLab 16.11.1) on 2024-05-02 at 15:00 UTC+2
As NULL value is always different for the UNIQUE constraint, we must use an EXCLUDE constraint with a COALESCE for when the parent is NULL.