Another solution could be:

SELECT
    COUNT(DISTINCT products_never_sold)*100.0/ COUNT(DISTINCT product_category) AS pct_product_categories_never_sold
FROM (

    SELECT
        product_category
        , (CASE WHEN SUM(units_sold) IS NULL THEN product_category END) AS products_never_sold
    FROM product_classes pc
    LEFT JOIN products p
        ON pc.product_class_id = p.product_class_id
    LEFT JOIN sales s
        ON s.product_id = p.product_id
    GROUP BY 1

)