Accept nil data when updating polymorphic to_one relationship

[Zooip]

Related to #656
Ensure that verified_params[:to_one].values[0] is present before calling [:id] and [:type]

This way the following request clear the polymorphic relationship in the same way as a non-polymorphic relationship.

PATCH /articles/1/relationships/author HTTP/1.1
Content-Type: application/vnd.api+json
Accept: application/vnd.api+json

{
  "data": null
}

Wasn't able to create a test case using defined test model since there is no to_one retaltionship.

[lgebhardt]

@Zooip Thanks!