Don't try to make a model for tiles without any buildings
- Prevents error: "Uncaught TypeError: Cannot read property 'vertexNormals' of undefined"
- Skips some unnecessary code and a worker thread

Error occurred on tiles like this:
http://vector.mapzen.com/osm/buildings/15/9646/12317.json
Merge branch '0.2.0' into check-no-buildings
Merge pull request #118 from brianchirls/check-no-buildings

Don't try to make a model for tiles without any buildings