do not use resp if it fails, use 0 instead

[vaikas]

Addresses #3791

When send fails or response is nil, set the response code to 0.

Proposed Changes

• Seen while debugging 3791, does not fix the flakiness though.

Release Note

- 🐛 Fix bug
In cases where Filter sends a message and it fails or response is nil, it will panic because it uses it.

Docs

[pierDipi]

/lgtm

[lberk]

/lgtm
/approve

thanks for looking into this

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lberk, pierDipi, vaikas

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[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/mtbroker/filter/filter_handler.go	86.6%	86.9%	0.3

[grantr]

Good catch! Would a unit test help here? Looks like coverage didn't decrease and is already pretty high, but this was a case not previously covered.

[vaikas]

Yeah, working on it... :)