reconsider object on DELETE nullability

[acpana]

Do we want to emulate real admission requests for the DELETE operation such that object is the new object being admitted. It is null for DELETE operations.

https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#webhook-request-and-response

Originally posted by @ritazh in #2512 (comment)

[acpana]

Adding some context per an offline chat w @maxsmythe about this. Doing so (object == nil on DELETE) may break constraints that rely on the previous behavior in gatekeeper.