8150187: NPE expected if the system identifier is null for CatalogResolver

Reviewed-by: rriggs, lancea

jaxp/src/java.xml/share/classes/javax/xml/catalog/CatalogResolverImpl.java

@@ -51,6 +51,7 @@ final class CatalogResolverImpl implements CatalogResolver {
 
     @Override
     public InputSource resolveEntity(String publicId, String systemId) {
+        CatalogMessages.reportNPEOnNull("systemId", systemId);
         //Normalize publicId and systemId
         systemId = Normalizer.normalizeURI(Util.getNotNullOrEmpty(systemId));
         publicId = Normalizer.normalizePublicId(Normalizer.decodeURN(Util.getNotNullOrEmpty(publicId)));