Add NOT NULL constraint on project_id or namespace_id on internal_ids

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)). 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.

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.