If directory provided to readDir is empty still fire async callback

I ran into an issue when using `readDir()` on an empty directory: `callback` is never fired. This hangs my server.

To reproduce:

```javascript
var git = require('git-fs');

git(__dirname);

git.readDir('fs', 'empty', function () {
    console.log('Works!');
});
```

This pull request contains a simple fix.