Do not use implicitly nullable parameters

[sebastianbergmann]

This pull request only contains the necessary code change for sebastianbergmann/phpunit#5719, it does not update the ChangeLog or prepare a release in any other way. This is because I ran into the following problems.

The ChangeLog that there is a 2.1.0 release. But there is no 2.1.0 tag, the latest tag is 2.0.3.

Since the release of 2.0.3, support for PHP 7.2 was dropped. For PHPUnit 8.5, we need a version of phar-io/manifest that supports PHP 7.2.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.83%. Comparing base (6772927) to head (38cb02c).

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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #35   +/-   ##
=========================================
  Coverage     99.83%   99.83%           
  Complexity      195      195           
=========================================
  Files            40       40           
  Lines           589      589           
=========================================
  Hits            588      588           
  Misses            1        1           

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