[FIX] website_sale_comparison: Tuple index out of range

Some product template with attributes never have variants because
the attributes have the field 'create_variant' set to 'no_variant'
So "Tuple index out of range" was raised due to these kind of products.

opw:1911962

closes odoo/odoo#29428

addons/website_sale_comparison/controllers/main.py

@@ -26,7 +26,8 @@ class WebsiteSaleProductComparison(WebsiteSale):
                     continue
                 res.setdefault(cat_name, {}).setdefault(att_name, [' - '] * len(products))
                 val = product.attribute_value_ids.filtered(lambda x: x.attribute_id == var.attribute_id)
-                res[cat_name][att_name][num] = val[0].name
+                if val:
+                    res[cat_name][att_name][num] = val[0].name
         values['specs'] = res
         values['compute_currency'] = self._get_compute_currency_and_context()[0]
         return request.render("website_sale_comparison.product_compare", values)