Fix #185: when y value is null, convert it to 0.
Fix #185: when y value is null, convert it to 0.

EverlyWell/redash,pubnative/redash,EverlyWell/redash,ninneko/redash,easytaxibr/redash,easytaxibr/redash,chriszs/redash,imsally/redash,imsally/redash,hudl/redash,hudl/redash,crowdworks/redash,denisov-vlad/redash,EverlyWell/redash,alexanderlz/redash,jmvasquez/redashtest,imsally/redash,ninneko/redash,stefanseifert/redash,jmvasquez/redashtest,pubnative/redash,guaguadev/redash,pubnative/redash,chriszs/redash,guaguadev/redash,vishesh92/redash,jmvasquez/redashtest,denisov-vlad/redash,guaguadev/redash,easytaxibr/redash,M32Media/redash,useabode/redash,denisov-vlad/redash,rockwotj/redash,44px/redash,crowdworks/redash,stefanseifert/redash,moritz9/redash,alexanderlz/redash,getredash/redash,pubnative/redash,M32Media/redash,amino-data/redash,moritz9/redash,44px/redash,guaguadev/redash,useabode/redash,hudl/redash,easytaxibr/redash,akariv/redash,chriszs/redash,imsally/redash,akariv/redash,stefanseifert/redash,amino-data/redash,amino-data/redash,EverlyWell/redash,vishesh92/redash,alexanderlz/redash,chriszs/redash,getredash/redash,akariv/redash,ninneko/redash,44px/redash,useabode/redash,akariv/redash,moritz9/redash,getredash/redash,denisov-vlad/redash,jmvasquez/redashtest,M32Media/redash,44px/redash,getredash/redash,hudl/redash,ninneko/redash,rockwotj/redash,easytaxibr/redash,moritz9/redash,vishesh92/redash,rockwotj/redash,pubnative/redash,ninneko/redash,rockwotj/redash,jmvasquez/redashtest,akariv/redash,getredash/redash,guaguadev/redash,crowdworks/redash,denisov-vlad/redash,alexanderlz/redash,vishesh92/redash,stefanseifert/redash,crowdworks/redash,stefanseifert/redash,amino-data/redash,useabode/redash,M32Media/redash