return null from GetNamespaceURI, not the empty string

[ajnirp]

Fixes #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.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2442

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @jdm (or someone else) soon.

[Manishearth]

r+, waiting for Travis.

In the meantime, it might be interesting to try and find out why the tagName test failed and solve that too :)

[Ms2ger]

@wenderen, I pointed out some now-passing tests on critic, and picked a few small nits while I was there. Thanks for your PR!

[Ms2ger]

@wenderen, can you please squash the four commits and force-push, so I can merge? Thanks!

[ajnirp]

Dumb question: I just need to do git rebase -i HEAD~4, pick the most recent and squash the rest, right?

[Manishearth]

Yep. f (fixup) instead of squash is better, otherwise it clutters up the commit message