improve get_rating_count() for rating-less reviews

Should close #6839.

According to `woocommerce_enable_review_rating` &
`woocommerce_review_rating_required`, it will do a different DB query,
so that when reviews without ratings are allowed, it will do a query
that counts those comments as reviews.
Merge pull request #6850 from bryceadams/patch-6839

improve get_rating_count() for rating-less reviews