Fix "null" device model for Firefox OS Tablet UA

The UA for Firefox OS tablets will be something like `Mozilla/5.0 (Tablet; Android; rv:26.0) Gecko/26.0 Firefox/26.0`. The current test `if ((that.test(/tablet/i) && !that.test(/RX-34/i)) || that.test(/FOLIO/i))` assumes it's a Blackberry (I'm guessing), but since there's no match on `/playbook`, `device.model` is set to null. Which means you get a silly "null" class added to the `documentElement`.

This patch sets it to the empty string instead, which makes more sense. :)