CreatedResource serializer breaks when content_object is deleted
CLOSED - CURRENTRELEASE
The CreatedResource is cleaned up when the Task is deleted, but not when the object it represents is deleted. This raises a 500 when the Task is serialized.
Either the CreatedResourceSerializer should be able to handle content_object=None or the CreatedResource should be cleaned up when the resource is deleted.
Serialize deleted resources as null
closes #3277 https://pulp.plan.io/issues/3277