title: Analogue of "SELECT * WHERE city != null" in Mongoose
body: <p>The question is, how to find data in my db, where <code>city != null</code>?</p>
  
  <pre><code>       userdata.find({ /*???*/ }, function (err, results) {
           if (!err) console.log(results)
           ...
         });
  </code></pre>