Correct error when trying to get dataURL from Stage without layers

If you try to get the dataUrl from a stage whithout layers, the javascript engine will throw a `TypeError ` exception.

```javascript
var stage = new Kinetic.Stage({
    container: containerId,
    width: 578,
    height: 200
});

// TypeError
stage.toDataURL({
    callback: function(data){  },
    mimeType: 'image/png'
});
```

To fix this issue I just added a check to see if there was any layer on the stage. If there isn't any, then just return nothing (stop execution).