mapIndexed: Fix generic type problem

[passsy]

Fixes error type 'DartMutableList<bool>' is not a subtype of type 'Null'

see tests for details

[codecov]

Codecov Report

Merging #22 into master will increase coverage by 1.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   63.29%   64.46%   +1.17%     
==========================================
  Files          25       25              
  Lines        1196     1196              
==========================================
+ Hits          757      771      +14     
+ Misses        439      425      -14

Impacted Files	Coverage Δ
lib/src/extension/iterable_extension_mixin.dart	57.63% <100%> (+2.64%)	⬆️
lib/src/collection/set_empty.dart	61.11% <0%> (+5.55%)	⬆️

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