[FIX] l10n_fr_pos_cert: Ensure country set on company before opening POS

Before this fix, if the country is not set on the company
the loading of the session may stall at some point.

A check is added and will prevent to start a session if the country
is not set.

opw-2557989

closes odoo/odoo#79212

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>