Scicat returns empty lists for list fields that were not set

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