createTreeWalker and createNodeIterator should deal with null NodeFilter

LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html

Index: LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html
diff --git a/LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html b/LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html
index 3852d1866ad1effe07357f1ee29329eea24226c4..7ef69a98adc717a68984891c8466bca532701636 100644
--- a/LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html
+++ b/LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html
@@ -193,6 +193,14 @@ test(function ()
         testIteratorForwardAndBackward(iterator, expectedAll);
     });
 }, 'Test that the fourth argument (expandEntityReferences) is ignored.');
+
+test(function() {
+    var ni = document.createNodeIterator(document.body, 42, null);
+    assert_equals(ni.root, document.body);
+    assert_equals(ni.referenceNode, document.body);
+    assert_equals(ni.whatToShow, 42);
+    assert_equals(ni.filter, null);
+}, "Optional arguments to createNodeIterator should be optional (3 passed, null).");
 </script>
 </body>
 </html>