The add-on is bugged and it should be ensuring the users have an email address before attempting to send to them.
Having a user without an email isn't generally possible - you can't register without an email - but it is possible for an admin to remove an email address when editing them in the Admin CP and creating a user in the Admin CP does not require entering an email address.
As such, any code within XF that attempts to send emails, actually checks first whether they have an email address and if they don't, it silently skips them. That's the behaviour we'd expect from any add-ons being released here that send emails to users.