static List<String> getXMPBagOrSeqList(XMPSchema schema, String name) {
        List<String> ret = schema.getBagList(name);
        if (ret == null) {
            ret = schema.getSequenceList(name);
        }
        return ret;
    }
As of this writing, XMPSchema can contain bags or sequence lists
for some attributes...despite standards documentation.
JempBox expects one or the other for specific attributes.
Until more flexibility is added to JempBox, Tika will have to handle both.

@param schema
@param name
@return list of values or null
tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-pdf-module/src/main/java/org/apache/tika/parser/pdf/PDMetadataExtractor.java