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

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

@@ -0,0 +1,11 @@
+var r = "OK"
+
+fun foo(x: Int) {
+    return when (x) {
+        1 -> { r = "Fail" }
+    }
+}
+fun box(): String {
+    foo(0)
+    return r
+}