fix empty poi list

If the map component gets an empty array for `pois: []` its creates an empty white wrapper container.
This PR will eliminate this edge case :point_down: 
![screen shot 2014-11-12 at 3 26 56 pm](https://cloud.githubusercontent.com/assets/2614324/5018980/df4bba08-6a80-11e4-8d3b-6888722489c3.png)

Verify: 
Pass an empty array to pois:
```= ui_component('poi_maps/dynamic', properties: { width: 800, height: 360, zoom: 15, marker: true, latitude: 18.5201, longitude: -77.8658, pois: [], center: "18.5201,-77.8658" })```