title: Firebase childEventListener return value when path does not exist
body: <p>Reading the documentation, It seems childEventListener does not fire when the path does not exist. </p>
  
  <p>This is a problem since I want to display a message to the user that there is no data.</p>
  
  <p>I could add a valueEventListener like <a href="https://stackoverflow.com/questions/34530566/find-out-if-child-event-listener-on-firebase-completely-load-all-data">in this answer</a> but I'm limiting the query to the latest value i.e query.limitToLast() and a valueEventListener doesn't limitTolast but gets all the data in the path.</p>
  
  <p>Example, I have:</p>
  
  <pre><code>posts
  {
      $userid
      {
          $postid {
              post_content:content
              timestamp:1234567
          }
          $postid {
              post_content:content
              timestamp:1234567
          }
          $postid {
              post_content:content
              timestamp:1234567
          }
          $postid {
              post_content:content
              timestamp:1234567
          }
      }
  }
  </code></pre>
  
  <p>I'm only interested in the latest post so I do <code>firebaseRef.child(users).child(userid).limitToLast(1).addChildEventListener</code> but the user might not have posts yet and childEventListener does not fire in that case.</p>