[18.0][IMP] partner_event: avoid matching partner_id as attendee when email…

[cvinh]

… is shared

Steps to reproduce :

1. Install partner_event
2. Create an event with "Create Partners in registration" checked
3. Create a partner with an email address
4. Create a registration for that event with:
   • partner_id = the partner above
   • name = "Child Attendee" (a different person)
   • email = same email as partner_id (e.g. fallback when attendee has no email)
5. Check attendee_partner_id on the registration

Expected: attendee_partner_id is created with empty email
Got: attendee_partner_id = partner_id

[BhaveshHeliconia]

Looks Good!