[FIX] purchase_stock: get ID of buy_route only if it exists

# How to reproduce:
- Delete external ID "purchase_stock.route_warehouse0_buy"
- Try to create a Product

"purchase_stock.route_warehouse0_buy" was meant to be optional. We can only get the id after checking that it exist.

OPW-2972494

closes odoo/odoo#102872

X-original-commit: 1b7b4cdbee07c325753c1b9362414e8309ef8718
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>
Signed-off-by: DavidFesquet <dafr@odoo.com>

addons/purchase_stock/models/product.py

@@ -11,10 +11,9 @@ class ProductTemplate(models.Model):
 
     @api.model
     def _get_buy_route(self):
-        buy_route_id = self.env.ref('purchase_stock.route_warehouse0_buy', raise_if_not_found=False).id
-        buy_route = self.env['stock.route'].search([('id', '=', buy_route_id)])
+        buy_route = self.env.ref('purchase_stock.route_warehouse0_buy', raise_if_not_found=False)
         if buy_route:
-            return buy_route.ids
+            return self.env['stock.route'].search([('id', '=', buy_route.id)]).ids
         return []
 
     route_ids = fields.Many2many(default=lambda self: self._get_buy_route())