[FIX] sale: correctly set default fiscal position on invoice

default fiscal position of previously selected partner is not getting
cleared up when current selected partner doesn't have a default one

task: 2071608
closes: #37231

$XX

closes odoo/odoo#37728

X-original-commit: bce4bcc61196bbeafb41f77b7a95051b225faea6
Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>