Add constraint to ensure that vulnerability_statistics.traversal_ids cannot be empty

This is needed to ensure that no entry will be created with an empty vulnerability_statistics.traversal_ids after we run the Backfill

EDIT: As per !188170 (comment 2758390614) there are still invalid records and so for now we have added the constraint as NOT VALID.

Next steps are to figure out the cause of these invalid records, re-queue the backfill bbm, then add the validated constraint.