fix TestRetry nil pointer panic

[dprotaso]

Discovered this just now

/assign @izabelacg

[knative-prow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.41%. Comparing base (75762ca) to head (984a3e0).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #965      +/-   ##
==========================================
- Coverage   93.43%   93.41%   -0.02%     
==========================================
  Files          36       36              
  Lines        1249     1003     -246     
==========================================
- Hits         1167      937     -230     
+ Misses         69       53      -16     
  Partials       13       13              

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[izabelacg]

/lgtm