title: How to present empty relationship in JSONAPI?
body: <p>Looked at <a href="http://jsonapi.org/format/" rel="nofollow noreferrer">http://jsonapi.org/format/</a> but don't see any description about empty relationship format, for example:</p>
  
  <pre><code>{
    "type": "articles",
    "id": "1",
    "attributes": {
      "title": "Rails is Omakase"
    },
    "relationships": {
      "comments": {
          "data": []
        }
    }
  }
  </code></pre>
  
  <p>This <code>article</code> doesn't have <code>comments</code>, what is the correct way to present a empty relationship?</p>
  
  <p><code>"data": []</code> or <code>"data": null</code> or no <code>"relationships"</code> at all?</p>
  
  <p>Thanks!</p>