Holdingpen: Update Conflicts: collaboration missing

[ksachs]

Collaborations are not necessarily in the metadata.

Expected Behavior

Do we want an conflict if the metadata have no collaboration?
I don't think so.

Current Behavior

Collaboration is proposed to be added.
Which is misleading anyhow: The question is whether we want to keep the collaboration.

Example

1154733

[StellaCh]

Do not create a conflict when there's already a collaboration. Will be changed after @michamos 's return