serving: Updating placeholder k8s services fails with Service is invalid: spec.clusterIP: Invalid value: ""

/area API /kind process

What version of Knative?

0.13.x

Expected Behavior

Route reconciliation to successfully complete when updating placeholder k8s services.

Actual Behavior

Route reconciliation makes it as far as marking the ingress ready on the route. Reconciliation fails after Updating placeholder k8s services with ingress information. This is the event logged:

Event(v1.ObjectReference{Kind:"Route", Namespace:"knative-applications", Name:"pubsub-event-mapper", UID:"67bb5fd2-c794-48cf-af99-14a58f41d8c5", APIVersion:"serving.knative.dev/v1", ResourceVersion:"208378251", FieldPath:""}): type: 'Warning' reason: 'InternalError' Service "pubsub-event-mapper" is invalid: spec.clusterIP: Invalid value: "": field is immutable

Steps to Reproduce the Problem

We just followed the instructions in the docs to install knative-serving, knative-eventing, and in-memory-channel. A broker was installed in the knative-applications namespace by labeling the namespace with knative-eventing-injection=enabled and then a knative-serving Service was deployed to the namespace knative-applications. In our logs we saw the route reconciliation process fail.