[FIX] mass_mailing_event : get corret list of attendees

The list of attendees to contact for an event is wrong.

Steps to reproduce the error:
1-In the event app , have a canceled attendee
2-Send an Email to attendees
bug : the email will be sent to canceled attendees as well

The problem was originated from the domain given by the inherited
class, there was no condition on the state of the attendees.

opw-3240100

closes odoo/odoo#117499

Signed-off-by: Adrien Widart <awt@odoo.com>