[JS IR BE] Don't render null messages in Throwable

Set message property to 'undefined' to make Error.prototype.toString
skip it

js/js.translator/testData/box/expression/try/exceptionToString.kt

@@ -1,3 +1,4 @@
+// IGNORE_BACKEND: JS
 // EXPECTED_REACHABLE_NODES: 1298
 package foo
 
@@ -8,8 +9,10 @@ class MyException3: Throwable {
     constructor(m: String? = null) : super(m) {}
 }
 
-// TODO: add direct inheritors of Throwable:
-// - with cause only, in the primary constructor
+class MyException4(c: Throwable? = null): Throwable(c)
+class MyException5: Throwable {
+    constructor(c: Throwable? = null) : super(c) {}
+}
 
 fun check(e: Throwable, expectedString: String) {
     try {
@@ -21,22 +24,28 @@ fun check(e: Throwable, expectedString: String) {
 }
 
 fun box(): String {
-    check(Throwable(), "Throwable: null")
+    check(Throwable(), "Throwable")
     check(Throwable("ccc"), "Throwable: ccc")
     check(Throwable(Throwable("ddd")), "Throwable: Throwable: ddd")
-    check(Exception(), "Exception: null")
+    check(Exception(), "Exception")
     check(Exception("bbb"), "Exception: bbb")
     check(Exception(Exception("ccc")), "Exception: Exception: ccc")
-    check(AssertionError(), "AssertionError: null")
-    check(AssertionError(null), "AssertionError: null")
+    check(AssertionError(), "AssertionError")
+    check(AssertionError(null), "AssertionError")
     check(AssertionError("bbb"), "AssertionError: bbb")
     check(AssertionError(Exception("ccc")), "AssertionError: Exception: ccc")
-    check(MyException(), "MyException: null")
+    check(MyException(), "MyException")
     check(MyException("aaa"), "MyException: aaa")
-    check(MyException2(), "MyException2: null")
+    check(MyException2(), "MyException2")
     check(MyException2("aaa"), "MyException2: aaa")
-    check(MyException3(), "MyException3: null")
+    check(MyException3(), "MyException3")
     check(MyException3("aaa"), "MyException3: aaa")
 
+    check(MyException4(), "MyException4")
+    check(MyException4(AssertionError()), "MyException4: AssertionError")
+
+    check(MyException5(), "MyException5")
+    check(MyException5(MyException5()), "MyException5: MyException5")
+
     return "OK"
 }