I had the exact same problem a week and a half ago. In my case we received over 6,000 email alerts with blank reports in one hour! I opened a ticket with tech support but we were not able to figure out the cause.
I was initially troubleshooting replication problems between my primary and backup SEPM servers when the blank email alerts started. Tried numerous things like stopping services, rebooting, deleting the event notification rule, and finally acknowledging all notifications. Still not certain if any one of those things actually fixed the problem or if it just cleared up on its own.