Validate NOT NULL constraint on internal_ids sharding key

What does this MR do and why?

Validate NOT NULL constraint on internal_ids sharding key

Following !199480 (merged)

Addresses #451900 (closed) && #555251

Epic: &11670

• validate the NOT NULL (NOT VALID) constraint was added to the gitlab.com table

  

• 0 rows with NULL sharding key: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/42015/commands/129055

• ensure that NOT VALID has been removed from the check constraint definition (https://docs.gitlab.com/development/database/not_null_constraints/#not-null-constraints-on-large-tables)

  

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