Make sure invalid arrays return empty array instead of throwing notices.

```php
array(
	array(
		'ParentCategory' => array(
			'id' => '1',
			'name' => 'Lorem ipsum dolor sit amet',
			'parent_id' => '1'
		)
	)
)
```
Results in
 
    Undefined offset: 0
    public static function nest(array $data, $options = array()) {} 
    Line 1041

https://github.com/cakephp/cakephp/blob/master/lib/Cake/Utility/Hash.php#L1041

This makes sure it returns an empty array.