Fix PHP 8.4 deprecation about implicit nullable types

[Jean85]

Fixes #58

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.17%. Comparing base (d8cd42a) to head (11179bc).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #59   +/-   ##
=========================================
  Coverage     64.17%   64.17%           
  Complexity      459      459           
=========================================
  Files            62       62           
  Lines          1552     1552           
=========================================
  Hits            996      996           
  Misses          556      556           

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