[FIX] sale_coupon: expiration date

MAke sure to always set the expiration date of a coupon.

opw-2151284

closes odoo/odoo#41470

X-original-commit: 2e5a6699
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/sale_coupon/models/sale_coupon.py

@@ -46,6 +46,7 @@ class SaleCoupon(models.Model):
     ]
 
     def _compute_expiration_date(self):
+        self.expiration_date = 0
         for coupon in self.filtered(lambda x: x.program_id.validity_duration > 0):
             coupon.expiration_date = (coupon.create_date + relativedelta(days=coupon.program_id.validity_duration)).date()