[selectors-api] Why null as opposed to empty string (or either one) to resolve default namespace

[Boris Zbarsky]

The editor's draft changes the spec to require passing null instead of 
"" to resolve the default namespace.  I was just wondering why (it's a 
bit more of a pain to implement for me, and doesn't seem like a useful 
change, but maybe there's a reason I'm missing.

-Boris