backport release-0.19: Check if spec.retry is nil before dereferencing it. (#2052)

[tommyreddad]

Fixes #2051

Proposed Changes

• Check if spec.retry is nil before dereferencing it.

Release Note

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

[knative-prow-robot]

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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%	67.6%	9.6

[tommyreddad]

/retest

[zhongduo]

/lgtm

[zhongduo]

/hold
This issue is already fixed by the latest master code, can you make sure the logic are the same?

[tommyreddad]

I cherry-picked the commit, it is the same logic. You can check the original to see it is the same: #2052

[zhongduo]

/unhold

[zhongduo]

/retest

[tommyreddad]

/retest

[tommyreddad]

/retest

[zhongduo]

/lgtm

[zhongduo]

/retest

[tommyreddad]

/retest