Prevents admin js errors by ensuring taxons key in api response

Previously, a response from /api/taxons (typically from the autocomplete in admin) would not include the 'taxons' key if the ransack query didn't yield any results.

To reproduce, simply enter a bogus taxon name in the autocomplete field for a product in the product admin, and js errors will occur for every character typed.

![screen shot 2014-01-07 at 5 47 04 pm](https://f.cloud.github.com/assets/57874/1864144/1cd75ca0-77ee-11e3-8020-e7e8ddcbb550.png)