Make getAttributeNS return null instead of empty string

Make getAttributeNS return null instead of empty string when no match is found, like
getAttribute does. This is specified in whatwg DOM and matches Firefox:

http://dom.spec.whatwg.org/#concept-element-attributes-get

BUG=

Review URL: https://codereview.chromium.org/250993003

git-svn-id: svn://svn.chromium.org/blink/trunk@172706 bbb929c8-8fbe-4397-9dbb-9b2b20218538