title: Firebase queries not calling block when there are no results
body: <p>I'm using Firebase and I want to query to see if something exists. It gets called when a value is found, but the block does not get called when nothing is found. Is this expected behaviour? </p>
  
  <pre><code>    ref.queryOrderedByKey().queryEqualToValue(channelName).observeSingleEventOfType(.ChildAdded, withBlock: { snapshot in
          print("found channel:  \(snapshot.key)")
          }, withCancelBlock: { error in
          print(error.description)
      })
  </code></pre>
  
  <p>Have I done something wrong? Thanks</p>