API returns unset array fields as empty arrays

[jl-wynen]

Fields with an array type are always returned as arrays. Even when they have not been set. This causes conflicts between raw and derived datasets. E.g., downloading a raw dataset returns

inputDatasets = []
usedSoftware = []

These fields are not allowed for raw datasets. So this can cause validation errors in clients. See, e.g., SciCatProject/scitacean#226.

Similarly, other array fields such as relationships and sharedWith are returned as [] even if they have not been set during upload.

Is this intended behaviour? Should clients treat empty arrays equivalently to absent / none fields?

[bpedersen2]

Should probably get fixed in the ds type unification