OptionalWhenBraces: fix false positive with lambda which has no arrow

[t-kameyama]

Fixes #2567

[codecov-io]

Codecov Report

Merging #2568 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2568      +/-   ##
============================================
- Coverage     25.12%   25.12%   -0.01%     
  Complexity      392      392              
============================================
  Files           378      378              
  Lines          7386     7387       +1     
  Branches       1219     1221       +2     
============================================
  Hits           1856     1856              
- Misses         5399     5400       +1     
  Partials        131      131              

Impacted Files	Coverage Δ	Complexity Δ
...rturbosch/detekt/rules/style/OptionalWhenBraces.kt	38.46% <0.00%> (-3.21%)	1.00 <0.00> (ø)

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