Data in relationships should be nullable

[nictas]

This is regarding the ToOneRelationship class: In it, the data field is not marked as @Nullable. However, when running cf curl /v3/organizations/<GUID>/domains/default I got the following response (note the relationships/organization field):

{
   "guid": "<GUID>",
   "created_at": "2016-10-06T08:30:16Z",
   "updated_at": "2019-03-04T11:12:14Z",
   "name": "<NAME>",
   "internal": false,
   "metadata": {
      "labels": {},
      "annotations": {}
   },
   "relationships": {
      "organization": {
         "data": null
      },
      "shared_organizations": {
         "data": []
      }
   },
   "links": {
      "self": {
         "href": "https://<API>/v3/domains/<GUID>"
      },
      "route_reservations": {
         "href": "https://<API>/v3/domains/<GUID>/route_reservations"
      }
   }
}