PHP 8.4 deprecation warnings

[Xnopyt]

This fixes a deprecation warning thrown by this library when running on PHP 8.4 with E_DEPRECATED reporting enabled

Phlib\JobQueue\Exception\JobRuntimeException::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead

Unit tests are still reporting a deprecation as there are some caused by the phlib/beanstalk library, these will be fixed by phlib/beanstalk#40

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.46%. Comparing base (8b6a4a9) to head (fe684bf).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #13   +/-   ##
=========================================
  Coverage     63.46%   63.46%           
  Complexity      106      106           
=========================================
  Files            10       10           
  Lines           375      375           
=========================================
  Hits            238      238           
  Misses          137      137           

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