OAIHarvest: resumptionToken fix

• FIX Fixes the parsing of resumptiontoken in incoming OAI-PMH XML which could fail when the resumptiontoken was empty.

Tested-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>