Fix key error problem with RLP Harvester

For the RLP Harvester there were some problems left with filling the
author fields. Sometimes the replacement field is not set or the author
email field is missing completely. For that reason I have added null
checks.

Berlin CKAN harvesting works fine so far, no errors during the process.
Hence, I have activated the plugin.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>