fixed problem with null values in string colums
fixed problem with null values in string colums

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