Trigger can pass `duck.WaitForResourceReady` on GKE without subscriber being deployed yet

[cardil]

Describe the bug
We have been hit by upgrade tests, which hadn't passed duck.WaitForResourceReady on trigger with subscriber that haven't deployed yet.

See related #5441 for description.

The code passes duck.WaitForResourceReady on GKE, but failed (properly) on OpenShift.

Expected behavior
Trigger shouldn't report Ready without subscriber being deployed.

To Reproduce
#5443

Knative release version
TBD

[cardil]

This isn't yet fixed in eventing, the #5443 still fails. Eventing needs updated knative/pkg to fix this.

[cardil]

/reopen