[FIX] sale: no show single novariant attribute in configurator

A product with single novariant attribute is normally not shown.

But in the configurator modal it was shown and then added in the product
description when doing exactly the same scenario (add the product) than
without the configurator.

With this changeset, we add is_single attribute and only add a no_create
variant if it is not a single value or a custom value.

opw-2494331

closes odoo/odoo#69411

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>