Allow returning null result

[greg0ire]

This can and does happen with Symfony's phpunit-bridge listener. Apart
for that, this property is not guaranteed to be not null (there is a
setter and no constructor requirement), so the return type hint should
reflect that.
See symfony/symfony#26861

[codecov-io]

Codecov Report

Merging #3087 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #3087      +/-   ##
============================================
+ Coverage     80.89%   80.92%   +0.02%     
- Complexity     2897     2898       +1     
============================================
  Files           108      108              
  Lines          7591     7591              
============================================
+ Hits           6141     6143       +2     
+ Misses         1450     1448       -2

Impacted Files	Coverage Δ	Complexity Δ
src/Framework/TestCase.php	67.37% <100%> (+0.24%)	305 <2> (+1)	⬆️

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[greg0ire]

Here is where the call happens: https://github.com/symfony/symfony/blob/9a999553a5495a23ea3be4271ecdbe89a9c64bba/src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php#L182

[sebastianbergmann]

Merged manually, thanks.