[-] MO : Followup : don't send emails for empty carts

"Cancelled cart" must only apply to carts which are not empty.

We have also changed the "ps_customer" table join to simplify the SQL request (RIGHT JOIN instead of LEFT JOIN + WHERE condition)

This Pull Request completes these ones:

- https://github.com/PrestaShop/PrestaShop/pull/482
- https://github.com/PrestaShop/PrestaShop/pull/484