title: What is returned from Mongoose query that finds no matches?
body: <p>I'm a little confused reading the Mongoose documentation. </p>
  
  <p>If I run a query in mongoose which matches no documents in the collection, what are the values of <code>err</code> and <code>results</code> in the callback function <code>callback(err, results)</code>? I just don't know what Mongoose considers an "error". As a mathematician, returning the empty set (i.e. <code>results</code> array empty) seems perfectly valid and shouldn't be an "error" - the query executed fine, there was just no matching documents. On the other hand, some may consider it an "error". From mongoose docs, either:</p>
  
  <ol>
  <li><code>err</code> = null, <code>results</code> = []</li>
  <li><code>err</code> = null, <code>results</code> = null</li>
  <li><code>err</code> = error document, <code>results</code> = null</li>
  </ol>