[FIX] calendar: accepting meeting invite from email

Behavior prior to the fix:

When accepting a meeting invite from the invitation email, if not logged
in, an Internal Server Error is received.  Additionally, if already
logged in, an Internal Server Error is also received (albeit for a
different reason)

Behavior after the fix:

Accepting (or declining) the meeting invite works and shows the
accepted/declined meeting.

opw-2366173

closes odoo/odoo#60556

Related: odoo/enterprise#14359
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>