Do not special case null for URLUtils attributes

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

 description('Test setting the pathname attribute of the URL in HTMLAnchorElement.');
 
 var a = document.createElement('a');
 
 debug("Set pathname that starts with slash");
 a.href = "https://www.mydomain.com/path/testurl.html?key=value";
 a.pathname = "/path name";
 shouldBe("a.href", "'https://www.mydomain.com/path%20name?key=value'");
 
 // IE8 throws an "Invalid URL" exception.
@@ skipping 14 matching lines @@
 shouldBe("a.href", "'https://www.mydomain.com//path%23name?key=value'");
 } catch(e) {
 debug("Exception: " + e.description);
 }
 
 debug("Set a pathname containing .. in it");
 a.href = "https://www.mydomain.com/path/testurl.html?key=value";
 a.pathname = "/it/../path";
 shouldBe("a.href", "'https://www.mydomain.com/path?key=value'");
 
-// IE8 converts null to "null", which is not the right thing to do.
 debug("Set pathname to null");
 a.href = "https://www.mydomain.com/path/testurl.html?key=value";
 a.pathname = null;
-shouldBe("a.href", "'https://www.mydomain.com/?key=value'");
+shouldBe("a.href", "'https://www.mydomain.com/null?key=value'");
 
 debug("Set pathname to empty string");
 a.href = "https://www.mydomain.com/?key=value";
 a.pathname = "";
 shouldBe("a.href", "'https://www.mydomain.com/?key=value'");
 
 // The expected behavior should change when the character table is updated.
 // IE8 considers this URL as valid.
 debug("Set pathname that includes illegal characters to URL that contains illegal characters.");
 a.href = "https://www.my|d[]()omain.com/path/testurl.html?key=value";
@@ skipping 10 matching lines @@
 debug("Exception: " + e.description);
 }
 
 // IE8 allows setting the pathname, for non-hierarchial URL.
 // It is not supposed to allow that per
 // http://dev.w3.org/html5/spec/infrastructure.html#url-decomposition-idl-attributes .
 debug("Set pathname to a URL with non-hierarchical protocol");
 a.href = "tel:+1800-555-1212";
 a.pathname = "the-path";
 shouldBe("a.href", "'tel:+1800-555-1212'");