[FIX] point_of_sale: prevent ValueError when tip product is deleted

"ValueError: No record found for unique ID point_of_sale.product_product_tip.
It may have been deleted." is generated when user delete 'Tips' from the product
and try to enable tip payment from the POS setting.

Step to produce:
1. Install point_of_sale
2. Open product and delete 'Tips' product
3. Open POS > Configure > Setting
4. Enable Tips in payment > select any product in tip product
5. click save >>> Traceback generated

This commit solves the above issue by passing 'raise_if_not_found=False'
to ensure it does not throw an error when the tip product is not found.

sentry-4232555961

closes odoo/odoo#124435

X-original-commit: b3675accd7d4ddfe97712060cda1259cf0b131f6
Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>
Signed-off-by: ANSARI MAHAMADASIF (maan) <maan@odoo.com>

addons/point_of_sale/models/pos_config.py

@@ -246,7 +246,7 @@ class PosConfig(models.Model):
     @api.depends('iface_tipproduct')
     def _compute_tip_product_id(self):
         for pos_config in self:
-            pos_config.tip_product_id = self.env.ref("point_of_sale.product_product_tip")
+            pos_config.tip_product_id = self.env.ref("point_of_sale.product_product_tip", raise_if_not_found=False)
             if not pos_config.tip_product_id:
                 pos_config.tip_product_id = self.env['product.product'].search([('default_code', '=', 'TIPS')], limit=1)