RFR (JDK 13/java.xml) 8216408: XMLStreamWriter setDefaultNamespace(null) throws NullPointerException

[Joe Wang]

Please review a fix for XMLStreamWriter.setDefaultNamespace that throws 
NullPointerException when the value is null.

JBS: https://bugs.openjdk.java.net/browse/JDK-8216408
webrevs: http://cr.openjdk.java.net/~joehw/jdk13/8216408/webrev/
Thanks,
Joe