Fix the handling of optional values in XML.
Fix the handling of optional values in XML.

Signed-off-by: Raphaël Barrois <raphael.barrois@polyconseil.fr>
(cherry picked from commit dea14763243418b2d7b3a10f664c79bbbfebe3c2)

Signed-off-by: Burak Arslan <84007f2f4c33c2a97a33044a3070a1e988bc918f@arskom.com.tr>

martijnvermaat/rpclib,arskom/spyne,arskom/spyne,martijnvermaat/rpclib,arskom/spyne,martijnvermaat/rpclib