Abide by first rule of structural schemas

[jcrossley3]

A non-empty type for the root is required per https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/#specifying-a-structural-schema

Otherwise, the following status condition appears in the CRD on a 1.16+ cluster:

• lastTransitionTime: "2020-03-17T16:58:54Z" message: 'spec.validation.openAPIV3Schema.type: Required value: must not be empty at the root' reason: Violations status: "True" type: NonStructuralSchema

[knative-prow-robot]

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