Do not format discount with currency if there is no currency

Closes #13531
(grafted from b35f96c592acdb738b2e2328dd274178514c0b90)

--HG--
branch : 7.0

modules/sale_discount/sale.py

@@ -133,8 +133,12 @@ class Line(metaclass=PoolMeta):
         if not rate or rate % Decimal('0.01'):
             amount = self.on_change_with_discount_amount()
             if amount:
-                return lang.currency(
-                    amount, self.sale.currency, digits=price_digits[1])
+                if self.sale and self.sale.currency:
+                    return lang.currency(
+                        amount, self.sale.currency, digits=price_digits[1])
+                else:
+                    return lang.format_number(
+                        amount, digits=price_digits[1], monetary=True)
         else:
             return lang.format('%i', rate * 100) + '%'