Stay on posts.index route when no posts exist

Currently, if there are no posts in the database, navigating to the Content screen results a URL of `/ghost/ember/undefined` as shown below.

This changes the behavior to stay on the posts.index route instead of transiting to posts.post with an undefined model.

Current:
![undefined](https://cloud.githubusercontent.com/assets/214142/3386832/f6f9e5d4-fc77-11e3-9a7b-f47be73b12f0.png)

Proposed:
![fixed](https://cloud.githubusercontent.com/assets/214142/3386911/af2fa4ae-fc78-11e3-8a60-7ca9c19da9a9.png)