[FIX] website_sale_comparison: no compare variant=False

When comparing product, there was already an exceptions that would not
list the attributes if they were only "create_variant=False" type.

But if we mix attribute create_variant False or not on a product, we
could get an error.

With this changeset, create_variant=False attribute are ignored also
when mixed with create_variant=True attributes.

opw-1946361
closes #31680

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