[FIX] mail: prevents access error when portal users opens a channel link

Before this commit, since https://github.com/odoo/odoo/pull/83574

,
attempting to join a call through the channel guest page while being
logged as a portal user would give an acess error.

This commit fixes this issue

closes odoo/odoo#87995

X-original-commit: e6a637dc
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>