CastToNullableType: allow casting null keyword

[dzirbel]

Fix a false positive for CastToNullableType for casting a literal null to a nullable type. This can be necessary when providing null as a function argument where the incoming type is parameterized. In this case using e.g. null as? String is less meaningful and generates a compiler warning that the cast cannot succeed.

[codecov]

Codecov Report

Merging #4907 (7f51415) into main (5baf1f6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #4907   +/-   ##
=========================================
  Coverage     84.83%   84.83%           
- Complexity     3511     3512    +1     
=========================================
  Files           497      497           
  Lines         11548    11549    +1     
  Branches       2138     2139    +1     
=========================================
+ Hits           9797     9798    +1     
  Misses          686      686           
  Partials       1065     1065           

Impacted Files	Coverage Δ
...arturbosch/detekt/rules/bugs/CastToNullableType.kt	87.50% <100.00%> (+0.83%)	⬆️

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[BraisGabin]

Thanks for the fix!