Currently `internal_ids` table doesn't have any records where both `project_id` and `namespace_id` is NULL (in Gitlab.com instance (internal link - [query plan](https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/27047/commands/84094))). But there is no NOT NULL constraint defined in database to ensure this presence, so we have to add a new constraint following our [guide](https://docs.gitlab.com/ee/development/database/not_null_constraints.html#add-a-not-null-constraint-to-an-existing-column). Also for now `internal_ids` table is exempted from `allowed_to_be_missing_not_null` check in `sharding_key_spec.rb`. On introducing this constraint, we can remove this exemption as well.