[FIX] account_edi_facturx: avoid crash when no edi attachment

When we get the values to embed to pdf, these values can be False if
there is no edi attachment on the account_move so try to assign something
to 'values' will fail.

Now there is a check before assignment.

opw-2526280

closes odoo/odoo#70613

Signed-off-by: Josse Colpaert <jco@openerp.com>