Report Unit returned by method implementations in interfaces

[3flex]

Closes #3099

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@94b5219). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #3108   +/-   ##
=========================================
  Coverage          ?   79.44%           
  Complexity        ?     2596           
=========================================
  Files             ?      437           
  Lines             ?     7818           
  Branches          ?     1484           
=========================================
  Hits              ?     6211           
  Misses            ?      819           
  Partials          ?      788           

Impacted Files	Coverage Δ	Complexity Δ
...rbosch/detekt/rules/style/optional/OptionalUnit.kt	80.95% <100.00%> (ø)	24.00 <3.00> (?)

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