Don't render/cache tiles with no features

We're not currently checking if [the return](https://github.com/openlayers/ol3/blob/c17424deec022c0cbc77bb2705e2b736b0925833/src/ol/renderer/canvas/canvasvectorlayerrenderer.js#L429-L431) from `getFeaturesObject` is empty.  If it is always empty in this loop, we don't need to add the `key` to `tilesOnSketchCanvas`.  If it doesn't appear there, it doesn't add to the rendering work or get cached.