Exit when there's nothing for d3 to render.
Exit when there's nothing for d3 to render.

Thanks to @alanshaw for the hint.

Close issue #74.

wzrdtales/david-www,alanshaw/david-www,JoseRoman/david-www,caktux/david-www,ExC0d3/david-www,JoseRoman/david-www,ecomfe/david-www,ExC0d3/david-www,ecomfe/david-www,80xer/david-www,caktux/david-www,80xer/david-www,wzrdtales/david-www