fixes method to return a falsy value if overlay type is unknown
fixes method to return a falsy value if overlay type is unknown

nuxcode/cartodb,raquel-ucl/cartodb,nuxcode/cartodb,CartoDB/cartodb,thorncp/cartodb,UCL-ShippingGroup/cartodb-1,splashblot/dronedb,future-analytics/cartodb,bloomberg/cartodb,splashblot/dronedb,thorncp/cartodb,UCL-ShippingGroup/cartodb-1,CartoDB/cartodb,bloomberg/cartodb,bloomberg/cartodb,dbirchak/cartodb,dbirchak/cartodb,nyimbi/cartodb,bloomberg/cartodb,DigitalCoder/cartodb,thorncp/cartodb,DigitalCoder/cartodb,dbirchak/cartodb,splashblot/dronedb,raquel-ucl/cartodb,nuxcode/cartodb,UCL-ShippingGroup/cartodb-1,thorncp/cartodb,DigitalCoder/cartodb,future-analytics/cartodb,codeandtheory/cartodb,future-analytics/cartodb,DigitalCoder/cartodb,UCL-ShippingGroup/cartodb-1,raquel-ucl/cartodb,DigitalCoder/cartodb,CartoDB/cartodb,bloomberg/cartodb,splashblot/dronedb,CartoDB/cartodb,nuxcode/cartodb,thorncp/cartodb,future-analytics/cartodb,future-analytics/cartodb,raquel-ucl/cartodb,splashblot/dronedb,codeandtheory/cartodb,UCL-ShippingGroup/cartodb-1,dbirchak/cartodb,nyimbi/cartodb,codeandtheory/cartodb,raquel-ucl/cartodb,nyimbi/cartodb,nuxcode/cartodb,CartoDB/cartodb,nyimbi/cartodb,codeandtheory/cartodb,dbirchak/cartodb,codeandtheory/cartodb,nyimbi/cartodb