title: Ignoring an object in struct is nil and not when it's an empty array
body: <p>Is it possible to only use omitempty when an object is nil and not when it's an empty array?</p>
  <p>I would like for the JSON marshaller to not display the value when an object is nil, but show <code>object: []</code> when the value is an empty list.</p>
  <pre class="lang-json prettyprint-override"><code>objects: nil
  
  {
    ...
  }
  </code></pre>
  <pre class="lang-json prettyprint-override"><code>objects: make([]*Object, 0)
  
  {
    ...
    &quot;objects&quot;: []
  }
  </code></pre>