Do not throw an exception when a when-statement with no else makes no match

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

@@ -0,0 +1,14 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+
+  val u: Unit = when(En.A) {
+    En.A -> {}
+    En.B -> {}
+  }
+
+  return "OK"
+}