RFR (JAXP) 8150187: NPE expected if the system identifier is null for CatalogResolver

[huizhe wang]

Hi,

Please review a patch that adds null-check to systemId.

JBS: https://bugs.openjdk.java.net/browse/JDK-8150187
webrevs: http://cr.openjdk.java.net/~joehw/jdk9/8150187/webrev/

Thanks,
Joe