fix: jest-worker can return null for getStdout()

[SimenB]

Summary

A null check was added in #8045 (comment). However, it most certainly can be null, and the type definition for WorkerInterface was wrong. jest-runner even guards against it: https://github.com/facebook/jest/blob/1fd565168dbf5b49780df959a214148eb35d32db/packages/jest-runner/src/index.ts#L113-L114

Fixes #8078

Test plan

Not really a testable change since the unit tests mock out child_process.fork. However, the type is now corrected.

[thymikee]

Looks good! There's one more "stdout is null" but in that case it really shouldn't be null, unless Node introduces a regression :P

[SimenB]

There's one more "stdout is null" but in that case it really shouldn't be null

Right, I left it alone on purpose

[codecov-io]

Codecov Report

Merging #8083 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8083      +/-   ##
==========================================
+ Coverage   62.33%   62.36%   +0.02%     
==========================================
  Files         263      263              
  Lines       10334    10330       -4     
  Branches     2500     2497       -3     
==========================================
  Hits         6442     6442              
+ Misses       3316     3314       -2     
+ Partials      576      574       -2

Impacted Files	Coverage Δ
...ages/jest-worker/src/workers/ChildProcessWorker.ts	85.71% <ø> (+6.46%)	⬆️
...kages/jest-worker/src/workers/NodeThreadsWorker.ts	87.23% <ø> (ø)	⬆️
packages/jest-worker/src/types.ts	100% <ø> (ø)	⬆️

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[github-actions]

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