Fix getUsefulTabs to handle cases where the first sample in a thread …

[julienw]

…is null

@padenot shared this profile with me: https://profiler.firefox.com/public/4ne691385vj2vxs3gzrsvvpbnkcne7tm74qgpxr/marker-chart/?globalTrackOrder=0w2&thread=2&v=8

It's not possible to select the parent process track.

The issue is that the first sample is null, and so this fails in

profiler/src/selectors/per-thread/composed.js

Line 95 in 1386366

const stackIndex = ensureExists(samples.stack[0]);

I'm not sure how a sample can be null when coming from Gecko, possibly because of sanitization?
I know samply can do this too. As a result this fixes #4364 too.

Production / Deploy preview (click on the parent process)
Production / Deploy preview (click on WebCOOP track)

[codecov]

Codecov Report

Base: 88.64% // Head: 88.64% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (ddd6fe8) compared to base (4e5442e).
Patch coverage: 83.33% of modified lines in pull request are covered.

❗ Current head ddd6fe8 differs from pull request most recent head d704a9f. Consider uploading reports for the commit d704a9f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4484      +/-   ##
==========================================
- Coverage   88.64%   88.64%   -0.01%     
==========================================
  Files         284      284              
  Lines       25587    25591       +4     
  Branches     6882     6885       +3     
==========================================
+ Hits        22681    22684       +3     
- Misses       2701     2702       +1     
  Partials      205      205              

Impacted Files	Coverage Δ
src/selectors/per-thread/composed.js	96.87% <83.33%> (-3.13%)	⬇️

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[mstange]

Thank you!

[julienw]

Thanks for the review!