[FIX] website: allow "Customize theme" when CDN option enabled
[FIX] website: allow "Customize theme" when CDN option enabled

Traceback because the `_get_asset` function was called by the
`theme_customize` route without the `values` argument (which is not
necessary for the feature).

Closes https://github.com/odoo/odoo/issues/22516

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo