bindings: Don't throw a TypeError when 'null' is passed to nullable callback function

third_party/WebKit/Source/core/testing/CallbackFunctionTest.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [ExperimentalCallbackFunction] callback TestCallback = DOMString (DOMString message1, DOMString message2);
 [ExperimentalCallbackFunction] callback TestInterfaceCallback = void (HTMLDivElement divElement);
 [ExperimentalCallbackFunction] callback TestReceiverObjectCallback = void ();
 [ExperimentalCallbackFunction] callback TestSequenceCallback = sequence<DOMString> (sequence<long> numbers);
 
 interface CallbackFunctionTest {
     [RaisesException] DOMString testCallback(TestCallback callback, DOMString message1, DOMString message2);
+    [RaisesException] DOMString testNullableCallback(TestCallback? callback, DOMString message1, DOMString message2);
     [RaisesException] void testInterfaceCallback(TestInterfaceCallback callback, HTMLDivElement divElement);
     [RaisesException] void testReceiverObjectCallback(TestReceiverObjectCallback callback);
     [RaisesException] sequence<DOMString> testSequenceCallback(TestSequenceCallback callback, sequence<long> numbers);
 };