admin/kcidb_models: KCIDBCheckout.policy should be never be null

This is niche, I admit, but the constraints on the policy field of the KCIDBCheckout object mean that in the admin site, if you're creating a checkout by hand, you can leave it as empy

If you do this it "works", in that you don't get an error and a DB entry is created, but essentially an orphan and can't be referenced because the AuthorizedModelAdmin queryset is looking for the policy.