Add default value for null center.
Add default value for null center.

staltz/tilestream,zhm/tilestream,frankrowe/tilestream,frankrowe/tilestream,NolaMark/tilestream,simonpoole/tilestream,NolaMark/tilestream,HydroLogic/tilestream,HydroLogic/tilestream,simonpoole/tilestream,zhm/tilestream,makinacorpus/tilestream,mapbox/tilestream,staltz/tilestream,mapbox/tilestream,makinacorpus/tilestream