Add NOT NULL constraint validation migration for GroupImportState model
This is a follow up issue for the NOT NULL constraint that has been added in scope of !41663 (merged)
Add NOT NULL constraint validation migration for GroupImportState
model as per https://docs.gitlab.com/ee/development/database/not_null_constraints.html#validate-the-text-limit-next-release guidelines.
It should be added to the next release after merged !41663 (merged) (13.5/13.6)