[19.0] [FIX] website_sale_hide_no_variant_attributes: never block add to cart

[ivantodorovich]

Attributes that never create variants are hidden from the product page, but their values were still required to sell the product. When those values are excluded for some variants, no complete combination may be left, and the product cannot be sold at all.

When no complete combination exists, the product now falls back on the variant-defining values alone: for the Add to Cart check, and for the first combination used by the product page, the shop grid, the wishlist, the comparison pages and the backend configurator.

Steps to reproduce

1. Create a product with a Size attribute (Small, Large) that creates variants.
2. Add a Material attribute that never creates variants, with the single value Cotton, and set its "Exclude for" to Large.
3. Add a Finish attribute that never creates variants, with the single value Glossy, and set its "Exclude for" to Small.
4. Open the product page.

Current: "This product has no valid combination", no Add to Cart button.
Expected: both sizes can be selected and added to the cart.