Fix Implicitly marking parameter $param as nullable is deprecated

[cedric-anne]

This fixes deprecation warnings triggered in PHP 8.4.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.65%. Comparing base (ccbf597) to head (0795cd7).
Report is 5 commits behind head on 4.5.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.5     #673   +/-   ##
=========================================
  Coverage     98.65%   98.65%           
- Complexity     1907     1930   +23     
=========================================
  Files            71       71           
  Lines          5294     5294           
=========================================
  Hits           5223     5223           
  Misses           71       71           

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[phil-davis]

Note: these changes happened in master PR #637 commit 5c3a532

I double-checked by trying to forward-port these changes, and they all result in merge-conflicts of code that is already fixed in master.

[phil-davis]

Released in https://github.com/sabre-io/vobject/releases/tag/4.5.6