Adding an employee fails at notification, when one of the admins email is empty

[fibasile]

Describe the bug On sentry: https://sentry.fab.city/sentry/fablabsio/issues/658/?environment=production

To Reproduce Steps to reproduce the behavior:

1. Any action that involves mailing to the lab admins, in the case that one of the employees has an empty email

Expected behavior

• If the email for one employee is empty we should report it to the superadmin
• If the email for one employee is empty the mailers should not error out
• There's should be no employee without email

[fibasile]

In this case the invalid email contains @¹gmail.com as a domain address. This was not catched by the validation at registration I guess

[fibasile]

The email is now correctly reported as invalid during registration, screenshot taken on staging