node.namespaceURI should never be undefined

[micschro]

It seems the namespaceURI property of attribute nodes that use the default namespace is always undefined.
The spec states that it should be null instead:

The namespace URI of this node, or null if it is unspecified.

http://www.w3.org/TR/2000/REC-DOM-Level-2-Core-20001113/core.html#ID-NodeNSname

This is a problem for us because it breaks our equals check for xml names (which compares the tagname and namespace).