Make sure one can OAI harvest repositories without sets

[jan356]

Some OAI harvestable sources do not have sets. It appears, that when you leave the field "OAI specific set id" empty in the "content source" tab of a collection you want to harvest from an external source, this is translated in the harvesting backend as "...&set=all&..." (see backend log file). This does not work for repositories not having sets.

Feature request 1: Can you please change this behavior so that when the field "OAI specific set id" is empty, it does not translate in the harvesting backend as "...&set=all&...", but translates as nothing/empty.

Feature request 2: The same holds for the "./dspace harvest" command: "-i" for OAI set id is a required parameter, please make it optional. Make sure it does not translate in the harvesting backend as "...&set=all&...", but translates as nothing/empty.

Example:
https://public.yoda.uu.nl/oai/oai?verb=ListRecords&set=all&metadataPrefix=oai_dc
is generated by the backend, with this repository it does not result in any records.

However, the same but without "...&set=all&..." does generate records:
https://public.yoda.uu.nl/oai/oai?verb=ListRecords&metadataPrefix=oai_dc