get_rating_html: Return a "not-rated" DIV instead of empty string

This will make the `li.product` same height.
In CSS, the `.non-rated` can be set to any color, opacity or visibility, (see the screenshot).

Alternatively, you can make a filter on the $rating_html.

Thank you!

![wc-rating-fix](https://cloud.githubusercontent.com/assets/1696330/5059810/b47f494e-6cfa-11e4-932b-5a53fcc80094.png)