[FIX] website_sale_loyalty: fix dictionary updating

After recent refactoring [1], sentry caught [2] `KeyError` on adding a record to
`res[coupon]` in `_get_claimable_and_showable_rewards` method.

Dictionary `res` may not have `coupon` and hence it leads to the error

https://github.com/odoo/odoo/blob/31b8151736b3382cca5601cd652b1d5482790af9/addons/website_sale_loyalty/models/sale_order.py#L198-L205

https://github.com/odoo/odoo/blob/31b8151736b3382cca5601cd652b1d5482790af9/addons/sale_loyalty/models/sale_order.py#L595

Fix it by checking `coupon in res` first.

[1]: https://github.com/odoo/odoo/commit/6525d5fba40c77ef9ed4445b8b9573550a1e2442

[2]: https://online.sentry.io/issues/4235487414

closes odoo/odoo#124616

X-original-commit: 57a00cae57116869f33e7267ad5ee84bd7600593
Signed-off-by: Ivan Elizaryev (iel) <iel@odoo.com>

addons/website_sale_loyalty/models/sale_order.py

@@ -215,5 +215,8 @@ class SaleOrder(models.Model):
                 if coupon.expiration_date and coupon.expiration_date < fields.Date.today():
                     continue
                 if points >= reward.required_points:
-                    res[coupon] |= reward
+                    if coupon in res:
+                        res[coupon] |= reward
+                    else:
+                        res[coupon] = reward
         return res