Do not special case null for URLUtils attributes

http://url.spec.whatwg.org/#api

The WebIDL attributes in the spec do not special case null and this
matches Firefox and IE.

BUG=304946

Review URL: https://codereview.chromium.org/26248003

git-svn-id: svn://svn.chromium.org/blink/trunk@159126 bbb929c8-8fbe-4397-9dbb-9b2b20218538