Don't fail in "brew cask list" with empty installation

At the moment `brew cask list` fails if no package is installed:
```sh
$ brew cask list
Error: nothing to list
$ echo $?
1
```

This makes it [quite hard](https://github.com/njam/osx-setup/commit/a4deda25e6ec9e43a1db9c14b7473b80a6c1b178) to use in another script.
Therefore I would suggest to just list *nothing* if no packages are installed:
```sh
$ brew cask list
$ echo $?
0
```

@mariansollmann fyi

What do you think?