Validate oauth_access_tokens#expires_in not null

What does this MR do and why?

• The background migration ran successfully on GPRD: !107701 (comment 1241450867)
• This migration prevent new invalid oauth access token records: !109047 (merged)
• Validating the not null constraint is the final step https://docs.gitlab.com/ee/development/database/not_null_constraints.html#validate-the-not-null-constraint-next-release
• #385374 (closed)

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