Relationship Object is too strict for requiring a `data` property according to the spec.

[mattpolzin]

Describe the bug
The JSON:API spec requires a Relationship Object to contain at leasts one of links, meta, and data. The current implementation always requires the data to be present.

Note that entirely omitting the relationship object or setting the data property to null are both supported currently, just not omitting the data property when the links or meta properties are present.

[mattpolzin]

This is fixed in v4.0.0-alpha.2 with the introduction of the MetaRelationship type. The new type can be used in the same places where ToOneRelationship and ToManyRelationship can be used.