Directories#all breaks when :local_root folder is missing for Fog::Storage::Local

Th local storage provider assumes the configured `:local_root` folder may not be present yet, for instance [here](https://github.com/fog/fog/blob/master/lib/fog/local/storage.rb#L87). However, `Directories#all` didn't. It would raise ("No such file or directory") when called before any object is created. This PR fixes this by returning an empty `Directories` instance instead.