CollectionInputFilter returns always valid for empty collections

this pull request adds a required flag to an InputFilterCollection

if you have nested arrays, it's impossible to enfoce at least one entry.

```php
$offers = array(
	array(
		'name' => 't-shirt',
		'prices' => array(
			// i need at least one price! 
                        // array('color' => 'black', 'price' => 149)
		),
	),
);
```

for example - this is very useful for mongodb documents.