RFR (JAXP): 8148350: Element.setAttributeNS() empty namespace does not throw exception

[Aleks Efimov]

Hi,
Please, help to review the fix for org.w3c.dom.Element#setAttributeNS 
method in JDK9:
According to w3.org documentation empty string cannot be used as a 
namespace name [1]. Therefore the setAttributeNS [2] should raise 
DOMException (NAMESPACE_ERR) when namespaceURI is empty string (similar 
to null value).
Webrev with proposed fix (regtest is included):
http://cr.openjdk.java.net/~aefimov/8148350/9/00/

Best Regards,
Aleksej

[1] https://www.w3.org/TR/xml-names/#iri-use
[2] 
https://docs.oracle.com/javase/8/docs/api/org/w3c/dom/Element.html#setAttributeNS-java.lang.String-java.lang.String-java.lang.String-