OptionalWhenBraces: fix false negative for nested when

[t-kameyama]

Fixes #4245

[codecov]

Codecov Report

Merging #4246 (3d3ab32) into main (deab0fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #4246   +/-   ##
=========================================
  Coverage     84.32%   84.32%           
  Complexity     3243     3243           
=========================================
  Files           470      470           
  Lines         10222    10223    +1     
  Branches       1787     1787           
=========================================
+ Hits           8620     8621    +1     
  Misses          663      663           
  Partials        939      939           

Impacted Files	Coverage Δ
...rturbosch/detekt/rules/style/OptionalWhenBraces.kt	93.75% <100.00%> (+0.41%)	⬆️

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