Unset taxonomy array if there are no terms. 

Stop displaying an empty dropdown in the media popup
I first thought about checking `if ( ! mediaTerms[key] )` in the JS when I thought about just hiding the it before creating the filter.
![media-taxonomy-empty-tags](https://f.cloud.github.com/assets/145887/1887748/ee776082-7a02-11e3-8297-8041902ea4ed.gif)