Allow relationship object omission if all relationships are optional

⚠️ Breaking-ish change ⚠️

If you had previously come to rely on the fact that the only way to allow omission of the relationships key was to create the Relationships = NoRelationships type alias, this change will be breaking. Still, because the JSON:API Spec allows for omission of the relationships key unconditionally, this change does bring the library more in line with the spec and I think this library's usability improves.