Validate NOT NULL for sharding key on sbom_occurrences_vulnerabilities

What does this MR do and why?

Validate the NOT NULL constraint for the sharding key on sbom_occurrences_vulnerabilities

• Async validation is verified to be complete on postgres.ai via \d+ sbom_occurrences_vulnerabilities (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 (re: https://docs.gitlab.com/ee/development/database/not_null_constraints.html#not-null-constraints-on-large-tables)

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)

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How to set up and validate locally

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