Add NOT NULL constraint validation migration for GroupImportState model

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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)