There has to be a way to un-set optional fields #770
Description
Currently, for most Option fields, such as zip, the field can be set, but not un-set (set to None) again.
That's because we treat undefined/null/not set in update requests as `ignore this field for the update.
it would be more correct, to distinguish between "set, but undefined/null" and "not set", as we do with the file upload ids (e.g. logo_file_upload_id) using the has_field helper.
However, it first would have to be aligned with frontend, what should be treated as what, to make that consistent