Validate NOT NULL constraint on issues.work_item_type_id

A NOT NULL constraint check was added on the issues table for the work_item_type_id column on new records. Once the issues table is backfilled for existing records, we should validate the constraint.

We have a callback before_validation :ensure_work_item_type on the issues model. We should change it to create only after the constraint is validated: before_validation :ensure_work_item_type, on: :create