CanBeNonNullable shouldn't consider abstract properties

[BraisGabin]

Related with #4677

[codecov]

Codecov Report

Merging #4686 (d243f24) into main (0be7775) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #4686   +/-   ##
=========================================
  Coverage     84.58%   84.58%           
  Complexity     3434     3434           
=========================================
  Files           492      492           
  Lines         11329    11329           
  Branches       2058     2058           
=========================================
  Hits           9583     9583           
  Misses          701      701           
  Partials       1045     1045           

Impacted Files	Coverage Δ
.../arturbosch/detekt/rules/style/CanBeNonNullable.kt	72.61% <100.00%> (ø)

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