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

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

@@ -0,0 +1,9 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String = when(En.A) {
+    En.A -> "OK"
+    En.B -> "Fail 1"
+}