do not render group when there are no children

**Before**

![topics_-_discourse_meta](https://f.cloud.github.com/assets/362783/912640/95c78d50-fe05-11e2-9ade-11865ec70b09.png)

**After**

![topics_-_zogstrip_s_discourse](https://f.cloud.github.com/assets/362783/912642/99c4bf7c-fe05-11e2-8b80-7c844687cc16.png)