[FIX] account_facturx: Don't fail if no partner is found

When searching for partners to fill the move, the refactoring done in
``find_partner`` for extracting the partner misses the case where no
record is found, as Form expects a recordset as entry, not an empty
list.

Note that on previous version, this error doesn't happen, as the
assignation was done only if the partner was found:

https://github.com/odoo/odoo/blob/e935cf9a/addons/account_facturx/models/account_invoice.py#L118



TT31272

closes odoo/odoo#74679

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>