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.

[phil-davis]

Similar to sabre-io/xml#271

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.17%. Comparing base (710f154) to head (d2581c7).
Report is 2 commits behind head on master.

❗ Current head d2581c7 differs from pull request most recent head ccbfb1a. Consider uploading reports for the commit ccbfb1a to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #234      +/-   ##
============================================
- Coverage     94.21%   94.17%   -0.05%     
  Complexity      261      261              
============================================
  Files            15       15              
  Lines           882      875       -7     
============================================
- Hits            831      824       -7     
  Misses           51       51              

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[phil-davis]

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