Add empty collection filter

New filter for doctrine, empty collection. When you make a copy of your object, matched element with this filter will be reset as an empty array collection. 

He is very ligh and usefull. 

NOTICE : I have rename the test class of DoctrineCollection accoding to the class name under test. (But i changed nothing)
This file docblock looks a bit off with the rest of the project, if that's fine by you, would you consider putting an `@author` tag on the class instead? Something like this:

```php
/**
 * @author johann <johann_27@hotmail.fr>
 */
class DoctrineEmptyCollection implements Filter
...
```