the feature is unavailable for Webi reports and it can be achieved using Publication and it is the easy way.
Still I would suggest you to go through the link below :
http://scn.sap.com/thread/320537
http://scn.sap.com/thread/3220011
This idea is already in SAP IDEA Place under review, Please vote for this
Do Not Send Email for Webi Scheduled Reports having no data : View Idea