[FIX] website_sale: fix disabled confirmation button for free order

- Go to website shop
- Add free product or discount to cart to make sure that total amount is 0
- Try to confirm order
It will not be possible because the "Confirm Order" button is disabled by
default and there is no way to enable it.

opw-2577089

closes odoo/odoo#74323

X-original-commit: c61349244ee09577c3e07e7b75da8a3c6134b0cf
Signed-off-by: Anh Thao PHAM <kitan191@users.noreply.github.com>

addons/website_sale/static/src/js/website_sale_payment.js

@@ -94,6 +94,16 @@ odoo.define('website_sale.payment', require => {
                 'change #checkbox_tc': '_onClickTCCheckbox',
             },
 
+            /**
+             * @override
+             */
+            start: function () {
+                this.$checkbox = this.$('#checkbox_tc');
+                this.$submitButton = this.$('button[name="o_payment_submit_button"]');
+                this._onClickTCCheckbox();
+                return this._super(...arguments);
+            },
+
             //--------------------------------------------------------------------------
             // Handlers
             //--------------------------------------------------------------------------