Fixed crash when relationship object doesn't have data key

[vvitto]

Regression bug appeared after upgrading the package to 2.0. It crashes when some relationship doesn't have data object, but with according to JSON API spec it should be possible such structure. For example:

"studio": {
  "links": {
    "self": "http://localhost:4000/api/rest/public/movies/1/relationships/studio",
    "related": "http://localhost:4000/api/rest/public/movies/1/studio"
  }
}