RFR [14/java.xml] 8016914: CoreDocumentImpl.setXmlVersion NPE

[Joe Wang]

Please review a fix to the NPE issue. This is part of the work on the 
XML declaration related issues.

JBS: https://bugs.openjdk.java.net/browse/JDK-8016914
webrev: http://cr.openjdk.java.net/~joehw/jdk14/8016914/webrev/
Thanks,
Joe