chore: use php-cs-fixer 3.51

[phil-davis]

php-cs-fixer 3.49 (or some recent minor release) wants to use the ? (nullable) syntax on optional parameters.
That syntax has been supported for quite a while since PHP https://www.php.net/manual/en/migration71.new-features.php

In the cases here, as well as the default value of the parameter being null, the ? explicitly allows the caller to pass the value null if they want.

This seems reasonable, I don't see how it can break any existing usage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.69%. Comparing base (11997fd) to head (c01b1e2).
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #271      +/-   ##
============================================
- Coverage     96.75%   96.69%   -0.07%     
  Complexity      115      115              
============================================
  Files            13       13              
  Lines           493      484       -9     
============================================
- Hits            477      468       -9     
  Misses           16       16              

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[phil-davis]

Has a minor complaint about project code coverage - strange report, the codecov/patch reports 100% of diff hit.

[phil-davis]

Bumped to use cs-fixer 3.51