Fix KT-10775 Empty when causes compilation exception.

Expression type for an empty 'when' is 'kotlin.Unit'.

compiler/testData/codegen/box/when/emptyWhen.kt

@@ -0,0 +1,7 @@
+enum class A { X1, X2 }
+
+fun box(): String {
+    when {}
+    when (A.X1) {}
+    return "OK"
+}