release-0.20: Add safeguards for Broker delivery spec nil pointer dereference

[tommyreddad]

Fixes delivery spec nil pointer dereference.

Proposed Changes

• Check spec before dereferencing in getPubsubRetryPolicy.
• Check spec before dereferencing in getPubsubDeadLetterPolicy.
• Add unit test for broker without delivery spec.

Release Note

- 🐛 Fixed a potential nil pointer dereference when using a GCP Broker.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tommyreddad

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[knative-metrics-robot]

The following is the coverage report on the affected files. Say /test pull-google-knative-gcp-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/trigger/trigger.go	58.0%	69.5%	11.5

[tommyreddad]

This implements some of the changes made in https://github.com/google/knative-gcp/pull/2030, but that change is too significant to cherry-pick into an older release. Namely, see the nil pointer checks in getPubsubRetryPolicy and getPubsubDeadLetterPolicy:

• https://github.com/google/knative-gcp/blob/0c8c969d46f01a0ce0d8fa6ba7ebda4528442145/pkg/reconciler/celltenant/target.go#L131-L137
• https://github.com/google/knative-gcp/blob/0c8c969d46f01a0ce0d8fa6ba7ebda4528442145/pkg/reconciler/celltenant/target.go#L159

[zhongduo]

/lgtm