XMLDBTransformer blows up Saxon

[Don Grodecki]

The XMLDBTransformer blows up Saxon, which does not like null URIs. The 
following fix seems to work, replacing null in the first parameter to 
addAttribute with "". - Don

                // Report result
                AttributesImpl attrs = new AttributesImpl();
                attrs.addAttribute("", XMLDB_QUERY_OID_ATTRIBUTE,
                        XMLDB_QUERY_OID_ATTRIBUTE, "CDATA", this.key);
                attrs.addAttribute("", XMLDB_QUERY_TYPE_ATTRIBUTE,
                        XMLDB_QUERY_TYPE_ATTRIBUTE, "CDATA", 
this.operation);
                attrs.addAttribute("", XMLDB_QUERY_RESULT_ATTRIBUTE,
                        XMLDB_QUERY_RESULT_ATTRIBUTE, "CDATA", result);