Allow optional when on Collector class

[samsonasik]

Allow when callable to optional, so can be like this:

        $data = [
            ' a ',
            ' b ',
            ' c ',
        ];

        $results = Collector::setUp($data)
            ->withTransform(fn (string $datum): string => trim($datum))
            ->getResults(); // ['a', 'b', 'c']

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d3c90d4) to head (f423102).

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Additional details and impacted files

@@             Coverage Diff             @@
##                main       #25   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        44        45    +1     
===========================================
  Files              4         4           
  Lines            120       122    +2     
===========================================
+ Hits             120       122    +2     

Flag	Coverage Δ
tests	100.00% <100.00%> (ø)

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