[FIX] Silence Passing null to class_exists() deprecation warning

[soltmar]

Since PHP 8.1 passing null to class_exists() is deprecated. I have re-factored SubstituteBindings::getClassName() to silence deprecation warning

This change could possibly be ported to other branches as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ab656aa) 66.15% compared to head (c3e368f) 66.15%.

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Additional details and impacted files

@@            Coverage Diff            @@
##                2.0     #568   +/-   ##
=========================================
  Coverage     66.15%   66.15%           
  Complexity      548      548           
=========================================
  Files            81       81           
  Lines          1764     1764           
=========================================
  Hits           1167     1167           
  Misses          597      597           

Files	Coverage Δ
src/Middleware/SubstituteBindings.php	100.00% <100.00%> (ø)

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[eigan]

Thanks 👍🏻