[FIX] mass_mailing: fix unsubscription_date when opt_out on create

Before this commit when creating a mailing contact with a mailing
list opt_out at creation the unsubscription_date wasn't set.
Indeed, from the mailing contact view the mailing list uses an
editable list that passes all the values at create even when not set
contrary to when one update the contact only the changed values are
passed. This commit fix this issue.

Task-3070852

closes odoo/odoo#106260

X-original-commit: d6758919
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Signed-off-by: Hoste Patrick (pko) <pko@odoo.com>