[FIX] product: exclude without attribute values
[FIX] product: exclude without attribute values

With the demo data:
- Go to Sale > Products > Products, search for 'Customizable Desk'
- Click on 'Configure Variants'
- For variant 'Legs: Steel', remove the attribute values for the
  'Conference Chair'
- Create a SO, add a line with 'Configure Product'
- Select the 'Customizable Desk' with 'Legs: Steel', add

The 'Conference Chair' is displayed in the 'Available Options', while it
shouldn't.

The methods `_get_own_attribute_exclusions` and
`_get_parent_attribute_exclusions` compute the exclusions based on the
attribute values. Since there is no values in our case, the exclusion is
not taken into account.

We add a specific case to search for exclusions without values.

opw-1945460
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo