validation error for Entity; relations; none is not an allowed value (type=type_error.none.not_allowed)

[sennierer]

we should either replace None with an empty array or allow None as a value for relations

@johannesliem would it break something in the frontend if we just allow None as a value for relations (which would mean that there is no relations key in the return format if there are no relations.

[johannesliem]

I just checked, in some places we check if the key exists, in several we assume the relations array exists (empty if no relations exist, but we do not check). So for now, it would be better to keep the empty array in the response format.