Handle null keypath in .valueForKeyPath

```coffee
_.valueForKeyPath(object, null) is object
```

This is to make atom's [`Config::get`](https://github.com/atom/atom/blob/master/src/config.coffee#L458) consistent with [`::observe`](https://github.com/atom/atom/blob/master/src/config.coffee#L351), in that a null/undefined `keyPath` corresponds to the entire object.