Scicat returns empty lists for list fields that were not set

[jl-wynen]

E.g., for a raw dataset, it returns

inputDatasets = []
usedSoftware = []

Neither are allowed to be set for raw datasets.

Scitacean flags this as an error (in the HTML repr and Dataset.make_upload_model). If this is intended behaviour of Scicat, we need to treat empty lists as None.