Mine too is blank and I ran it for over the last month.
Per this document:
http://www.symantec.com/business/support/index?page=content&id=TECH95538
Top Traffic Notifications Report
This report consists of a pie chart with relative bars that lists the group or subnet, and the number and percentage of notifications. It shows the number of notifications that were based on firewall rule violations that you configured as important to be notified about. The rules that are counted are those where you checked the Send Email Alert option in the Logging column of the Firewall Policy Rules list. You can view information for all, for the Traffic log, or for the Packet log, grouped by top groups or subnets.
Do you actively use the firewall and configure email alerts? If not, you won't see any data here, which is why I don't either.