Set `null` as empty value instead of `undefined`

[beachmachine]

Setting undefined for an attribute on an object that is going to be serialized as JSON will cause the attribute to be skipped from the JSON. I think this is not what is usually needed to empty values as most REST APIs will either throw an error (as a mandatory attribute is not in the API call) or will not update the attributes value.

Setting null as empty value instead of undefined solves this problem.