Update arc.js
Update arc.js

Fix for issue #1205.

I added a console.log while debugging this to verify that this null condition was being hit and that the charts still generated correctly after hitting this.
c3js/c3,alanhamlett/c3,WeaveTeam/c3,naver/billboard.js,c3js/c3,Aloomaio/c3,hds-lab/c3,alanhamlett/c3,aendrew/c3,hds-lab/c3,masayuki0812/c3,Kreozot/c3,pivotaltracker/c3,aendrew/c3,WeaveTeam/c3,Aloomaio/c3,mwilkows/c3,EspadaV8/c3,Kreozot/c3,c3js/c3,GerHobbelt/c3,mwilkows/c3,getconnect/c3,missingdays/c3,masayuki0812/c3,naver/billboard.js,benjyblack/c3,davidpoore/c3,missingdays/c3,davidpoore/c3,benjyblack/c3,misticini/c3,pivotaltracker/c3,c3js/c3,alanhamlett/c3,GerHobbelt/c3,getconnect/c3,EspadaV8/c3,naver/billboard.js,misticini/c3,naver/billboard.js