[FIX] l10n_fr_pos_cert: raise error when there is no order

Before this commit: if the user wanted to generate a
POS Inalterability Check statement, an IndexError was raised.
This commit raises a more understandable error.

opw-3237383

closes odoo/odoo#116386

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>