Deserializing a null belongsTo doesn't create an empty model

Somewhat similar to 1d3bbb0cbb9824088f7e411d4ca402ef6ec3a561, deserializing a `null` shouldn't create an empty model object.

I could see this being a tad bit controversial, but IMO, a `null` for an embedded `belongsTo` implies the lack of a foreign key or link, thus it should remain blank when the deserialized model is re-serialized.