[FIX] account_edi_ubl_cii: invoice lines could have multiple fixed taxes

An UBL Bis 3 xml can only contain one tax per invoice line. However,
there might be one or more ecotaxes on invoice lines in Belgium. Do not
block the user from generating the attachment in this case.

Note that PR https://github.com/odoo/odoo/pull/121494

 will properly
handle the fixed taxes upon generating UBL attachments.

opw-3318969

closes odoo/odoo#121854

X-original-commit: 68814f52
Signed-off-by: William André (wan) <wan@odoo.com>
Signed-off-by: Julien Van Roy <juvr@odoo.com>