title: loopback where field is null
body: <p>How can I query a field which is null? I get a a sql error</p>
  
  <p>filter=[where][category_id][eq][null] results in</p>
  
  <pre><code>SELECT WHERE category_id'null' ORDER BY id
  </code></pre>
  
  <p>using latest version of loopback and loopback-connector-mysql</p>