[FIX] website_sale: return sale.order object

Always return a `sale.order` object, otherwise calling method may crash
since they expect receiving such an object.

opw-761344

Manual forward-port of 0888602831 which has
been lost during normal one.

addons/website_sale/models/website.py

@@ -293,7 +293,7 @@ class Website(models.Model):
 
         else:
             request.session['sale_order_id'] = None
-            return None
+            return self.env['sale.order']
 
         return sale_order