not required not the same as nullable: true

[edeutsch]

I think in several places in the schema, we assumed that not required properties were equivalent to nullable: true
But according to JSON schema validation, this is not true. It appears that a non-required property that is not explicitly defined to be nullable: true can fail validation if it is null.

TODO: Check all properties and define them all as nullable: true or nullable: false to avoid this ambiguity.