[FIX] purchase: don't raise if standard route is deleted

New api regression: xmlid_to_res_id() had the flag to raise set to
false by default while ref() does the opposite. Since the intent
here was clearly to allow not finding the record (given the if
condition under the ref call), avoid crashing for a simple default.

addons/purchase/purchase.py

@@ -983,7 +983,7 @@ class ProductTemplate(models.Model):
 
     @api.model
     def _get_buy_route(self):
-        buy_route = self.env.ref('purchase.route_warehouse0_buy')
+        buy_route = self.env.ref('purchase.route_warehouse0_buy', raise_if_not_found=False)
         if buy_route:
             return buy_route.ids
         return []