A null value in any of the *_id attributes of host results in an incorrect error message.

Here is the JSON I PUT:

{
    "realm_id": null
}

And here is the response

{
  "error": {
    "message": "Resource host not found by id '1'" 
  }
}

If an *_id attribute is null then either it should be ignored or it should remove the association as applicable.