Validate NOT NULL on vulnerability_finding_signatures sharding key

What does this MR do and why?

Validate the NOT NULL constraint for the sharding key on vulnerability_finding_signatures

• Async validation is verified to be complete on postgres.ai via \d+ vulnerability_finding_signatures (no shareable link)
• No presence: true validation on the model because the column is also populated with a "BEFORE INSERT OR UPDATE" trigger.
  • Before being committed, the database will fill the column if its null.
• Applied pipeline:skip-check-migrations as reverting this migration is intended to be #no-op. Rolling back wouldn't restore structure.sql to its previous
  • Failed spec: https://gitlab.com/gitlab-org/gitlab/-/jobs/9071146005

References

• Organization Isolation and Sharding Keys: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/organization/isolation/
• Sharding Key (Cells) Development Guide: https://docs.gitlab.com/ee/development/cells/#defining-a-sharding-key-for-all-cell-local-tables
• Related to #493768 (closed)

MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

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