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 1.3.3 (GitLab 16.9.6) on 2023-04-25 at 18:00 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.