Add test for getting an empty grid (#50)
Add test for getting an empty grid (#50)

The tests succeed.

HydroLogic/Windshaft,manueltimita/Windshaft,wsw0108/Windshaft,HydroLogic/Windshaft,CartoDB/Windshaft,manueltimita/Windshaft,wsw0108/Windshaft,HydroLogic/Windshaft,wsw0108/Windshaft,CartoDB/Windshaft,manueltimita/Windshaft,CartoDB/Windshaft