filter the results of env_list

### What is the problem / feature ?

The `env_list` helper would return `['']` if it got an empty string as the env variable.  It seems more logical for it to return an empty list.

### How did it get fixed / implemented ?

Changed it to filter the return value.

### How can someone test / see it ?

Pull an empty env variable using `env_string` and see it's an empty list.

*Here is a cute animal picture for your troubles...*

![6cf185b86877d24966f36f2a7dd07a8b](https://cloud.githubusercontent.com/assets/824194/5500612/625a99be-8705-11e4-95ae-21a3d1ec4dca.jpg)