Hi,
With any database non-required fields will not exclude the entire entry with a simple query. If you leave a non-required field empty the fields needed to generate the lead will still create an entry and ID for the lead so it will count towards the total number of leads allowed.
It would need to filter the emails specifically for the count, but since it is for the entire list email or not it will count. hope that explains it clearer.
Best of luck,
Dan