Implement Stringer for nil podTracker

[evankanderson]

Proposed Changes

• Implement the Stringer interface for nil podTrackers. This sometimes causes spurious fuzzing failures when printing / comparing podTrackers.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04 ⚠️

Comparison is base (bde2f42) 86.23% compared to head (f886195) 86.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14147      +/-   ##
==========================================
- Coverage   86.23%   86.20%   -0.04%     
==========================================
  Files         199      199              
  Lines       14795    14798       +3     
==========================================
- Hits        12759    12756       -3     
- Misses       1733     1736       +3     
- Partials      303      306       +3     

Impacted Files	Coverage Δ
pkg/activator/net/throttler.go	87.87% <100.00%> (+0.07%)	⬆️

... and 2 files with indirect coverage changes

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[dprotaso]

Curious what kind of fuzzing are you doing?

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, evankanderson

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