fix: if the relationship is null the jsonapi output must present null

[elfin-sbreuers]

The JSONAPI spec expects a null value if a toOne relation is empty https://jsonapi.org/format/#fetching-relationships

The current implementation always returns

"relationships": { "relation": { "data": [] } }

even though it should return

"relationships": { "relation": { "data": null } }

This breaks JSONAPI copliant client code.