Do not special case null for URLUtils attributes

LayoutTests/fast/dom/HTMLAnchorElement/script-tests/set-href-attribute-protocol.js

 description('Test setting the protocol attribute of the URL in HTMLAnchorElement .');
 
 var a = document.createElement('a');
 
 debug("Basic test");
 a.href = "https://www.mydomain.com/path/";
 a.protocol = "http-foo";
 shouldBe("a.href", "'http-foo://www.mydomain.com/path/'");
 
 // IE8 throws "Invalid argument" exception.
@@ skipping 26 matching lines @@
 // IE8 throws "Invalid argument" exception.
 try {
 debug("Set a protocol that starts with ':'");
 a.href = "https://www.mydomain.com/path/";
 a.protocol = ":http";
 shouldBe("a.href", "'https://www.mydomain.com/path/'");
 } catch(e) {
 debug("Exception: " + e.description);
 }
 
-// IE8 converts null to "null", which is not the right thing to do.
 debug("Set protocol to null");
 a.href = "https://www.mydomain.com/path/";
 a.protocol = null;
-shouldBe("a.href", "'https://www.mydomain.com/path/'");
+shouldBe("a.href", "'null://www.mydomain.com/path/'");
 
 // IE8 throws "Invalid argument" exception.
 try {
 debug("Set protocol to empty string");
 a.href = "https://www.mydomain.com/path/";
 a.protocol = "";
 shouldBe("a.href", "'https://www.mydomain.com/path/'");
 } catch(e) {
 debug("Exception: " + e.description);
 }
 
 // Firefox 4 adds three slashes, unlike Safari and Chrome
 debug("Set protocol to http on malformed URL");
 a.href = "foo:??bar";
 a.protocol = "http";
 shouldBe("a.href", "'http:/??bar'");
 
 // IE8 keeps the protocol if it is 'c:'.
 debug("Set protocol to a URL which points to a local file");
 a.href = "c:\path";
 a.protocol = "f-oo";
 shouldBe("a.href", "'f-oo:path'");
 
 debug("Set protocol to undefined");
 a.href = "https://www.mydomain.com/path/";
 a.protocol = undefined;
 shouldBe("a.href", "'undefined://www.mydomain.com/path/'");