Converter:

Omit Unit return type
Refactor handling of Unit type: extract separate object UnitType

j2k/tests/testData/ast/enum/enumConstantIsNotNull.kt

@@ -1,6 +1,6 @@
 enum class E {
 FOO
-fun foo() : Unit {
+fun foo() {
 FOO.toString()
 }
 }