[Products] Show the downloadable files alert only when needed

[hichamboushaba]

In #11388, we added logic to show an alert when the user tries to make a product non-downloadable, it would be nice to have an additional check to show the alert only when there are existing downloadable files in the product.

Check this discussion for more details: #11388 (comment)

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.