title: Sharepoint REST API filter empty LookUp field
body: <p>I'd like to know how I can filter ListItems in SharePoint rest API (2013) on empty lookup.
  On my list items, there is a lookup (single value, not required) and i'd like to get all items where lookup is empty (blank).</p>
  
  <pre><code>_api/web/lists/getbytitle('MyList')/items?$select=Id&amp;$expand=MyLookUp/Id&amp;$filter=??
  </code></pre>
  
  <p>Does someone has a clue ? </p>
  
  <p>Thx</p>