return null from GetNamespaceURI, not the empty string

Fixes https://github.com/servo/servo/issues/3140

I tried changing the ini file for the Document-createElementNS.html test. I expected that changing the last three tests (the null / undefined / empty string namespace ones) to PASS would work, but apparently the tagName assert makes them fail. I guess that's a separate issue, so I left the ini file as it was.