OptionalUnit: fix false positive with 'else if'

[t-kameyama]

Fixed false positive with 'else if' :

fun <T> T.foo() {
    println(this)
}

fun test(i: Int, j: Int) {
    when (i) {
        0 -> println(1)
        else -> {
            if (j == 1) {
                println(2)
            } else if (j == 2) {
                println(3)
            } else if (j == 3) {
                println(4)
            }
            Unit
        }
    }.foo()
}

Related: #2886

[codecov]

Codecov Report

Merging #2888 into master will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master    #2888      +/-   ##
============================================
- Coverage     80.19%   80.18%   -0.02%     
  Complexity     2425     2425              
============================================
  Files           421      421              
  Lines          7368     7369       +1     
  Branches       1349     1349              
============================================
  Hits           5909     5909              
  Misses          761      761              
- Partials        698      699       +1     

Impacted Files	Coverage Δ	Complexity Δ
...rbosch/detekt/rules/style/optional/OptionalUnit.kt	77.77% <75.00%> (-1.77%)	26.00 <0.00> (ø)

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