XmlViewWriter.writeMeta throws NPE

[ceharris]

If the value associated with the meta in the view is null, we should be generating an empty element, just as we do for any other null value.

Suggest XmlViewWriter.writeMeta should be implemented as:

    if (event.getValue() == null) {
      writeEmptyElement(event.getName(),
          XmlViewConstants.META_NAMESPACE, XmlViewConstants.NULL_QNAME);
    }
    else {
      writeAttributeString(event.getName(),
          XmlViewConstants.META_NAMESPACE, event.getValue().toString());
    }