kt-fast-collections: fix array init performance bug

[multun]

Before

After

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.26%. Comparing base (b1f8892) to head (c6ac2c1).

Additional details and impacted files

@@            Coverage Diff            @@
##                dev    #6805   +/-   ##
=========================================
  Coverage     28.26%   28.26%           
  Complexity     2179     2179           
=========================================
  Files          1049     1049           
  Lines        130290   130290           
  Branches       2578     2578           
=========================================
  Hits          36820    36820           
  Misses        91981    91981           
  Partials       1489     1489           

Flag	Coverage Δ
core	79.44% <ø> (ø)
editoast	75.14% <ø> (ø)
front	8.62% <ø> (ø)
gateway	2.50% <ø> (ø)
railjson_generator	87.26% <ø> (ø)
tests	83.07% <ø> (ø)

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[eckter]

I can confirm it makes a difference 👍