How NOT to send mail when a variable is empty

[Manutenzione Sala Corciano]

https://help.trello.com/article/1192-creating-reports-with-butler

I've read this article and it's ok how to create variable by report and send it by mail.

But how can I do to DON'T send an email if after report the variable is empty?

For example I don't want send any mails if I have not any overdue task. 

[Manutenzione Sala Corciano]

I try to explain better:

I've this scheduled task:

"every day, create a report in variable "reportOverdue" with all cards with an overdue due date, and send an email notification to every member on the board with subject "Overdue" and message "Overdue Cards:\n{$reportOverdue}"

It works but it sends always an email everyday also if there's not overdue cards.

I would like that if there's no overdue cards it doesn't send any mail.