Tests failing with panic when a broker being created already exists.

[vaikas]

Describe the bug
As witnessed by several failing runs (one example here):
https://github.com/knative/eventing/runs/1677836817?check_suite_focus=true

Looks like we're trying to create a Broker that already exists and it fails:

--- FAIL: TestBrokerNamespaceDefaulting (2.27s)
    test_runner.go:153: namespace is : "test-broker-namespace-defaulting-c6rnm"
    broker_defaults_webhook_test.go:53: Updating defaulting ConfigMap attempt: 0
    broker_defaults_webhook_test.go:90: CM updated - new values: |
          clusterDefault:
            apiVersion: v1
            brokerClass: MTChannelBasedBroker
            kind: ConfigMap
            name: config-br-default-channel
            namespace: knative-eventing
          namespaceDefaults:
            test-broker-namespace-defaulting-c6rnm:
              brokerClass: MTChannelBasedBroker
              delivery:
                backoffDelay: PT0.5S
                backoffPolicy: exponential
                retry: 5
        
    broker_defaults_webhook_test.go:132: 
        	Error Trace:	broker_defaults_webhook_test.go:132
        	            				util.go:51
        	            				wait.go:211
        	            				wait.go:399
        	            				util.go:50
        	            				broker_defaults_webhook_test.go:112
        	Error:      	Expected nil, but got: &errors.StatusError{ErrStatus:v1.Status{TypeMeta:v1.TypeMeta{Kind:"Status", APIVersion:"v1"}, ListMeta:v1.ListMeta{SelfLink:"", ResourceVersion:"", Continue:"", RemainingItemCount:(*int64)(nil)}, Status:"Failure", Message:"brokers.eventing.knative.dev \"xyz-0\" already exists", Reason:"AlreadyExists", Details:(*v1.StatusDetails)(0xc0021758c0), Code:409}}
        	Test:       	TestBrokerNamespaceDefaulting
    k8s_events.go:66: EventListener stopped, 0 events seen
    test_runner.go:223: Export logs in "test-broker-namespace-defaulting-c6rnm" to "/home/runner/work/eventing/eventing/artifacts/pod-logs"
panic: runtime error: invalid memory address or nil pointer dereference [recovered]

A clear and concise description of what the bug is.

Expected behavior
Tests to work consistently.

To Reproduce
Steps to reproduce the behavior.

Knative release version
HEAD

Additional context
Add any other context about the problem here such as proposed priority

[vaikas]

fixed by #4716