Converter:

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

j2k/tests/testData/ast/assignmentExpression/nullability.kt

@@ -1,6 +1,6 @@
 import java.util.BitSet
 open class Foo() {
-open fun foo(o : BitSet?) : Unit {
+open fun foo(o : BitSet?) {
 var o2 : BitSet? = o
 var foo : Int = 0
 foo = o2?.size()!!