Hi,
I assume you are running the broadcasting using process chain, right?
You may use this program to know if zero record was fetched..Insert this in your process chain before broadcasting..
Check if zero records are fetched and terminate load in a process chain
OR
If you are using the SCHEDULE button in the portal, you may CHECK this setting so that if there is an added record that's the only time the email will be broadcasted..
Regards,
Loed