[IMP] tools, base, mail: use first found email in 'email_normalized'

PURPOSE

Be defensive when dealing with email fields, notably when having multi-emails
or email field containing an already-formatted email.

SPECIFICATIONS

When having multi-emails input in an email field, 'email_normalized' field is
currently 'False', as they expect the field to contain a single email. This
has several drawbacks

  * searching partners or fetching information based on emails does not work as
    most tool methods use 'email_normalized' which is False (see e.g.
    '_message_partner_info_from_emails', '_mail_find_partner_from_emails'
    or 'find_or_create');
  * blacklist is not available as it is based on 'email_normalized';
  * mass_mailing wrongly considers those emails as invalid and cancel their
    mail and related trace, as it tries to skip sending emails to invalid
    emails;

Be more defensive and use first found email in case of multi-emails field.
Other emails are ignored. It is already an improvement that does not break
flows in stable and allow more emails to be sent.

  before
  -> email: '"Raoul" <raoul1@raoul.fr>, raoul2@raoul.fr'
  -> email_normalized: False
  after
  -> email: '"Raoul" <raoul1@raoul.fr>, raoul2@raoul.fr'
  -> email_normalized: raoul1@raoul.fr

A side effect is that it helps finding back some partners, as indicated in
tests where less phantom partners are created.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474