Define optional/nullable fields convention (REST API + datamodel)

[slint]

There are three ways to describe a nullable/optional value right now:

• Field is not there at all
• Field is there but with a null value
• Field is there but with "empty" value ("" for strings, [] for lists)

Once we have a clear decision on this we can clearly reason about how to proceed... This affects REST API (and overall input) semantics also for when e.g. we want to "clear" a field.

[github-actions[bot]]

This issue was automatically marked as stale.