DDC-3056 null value in in criteria support

See DDC-3056 (http://www.doctrine-project.org/jira/browse/DDC-3056)

This *MAY* be a breakage, since the following API now works as expected:

```php
$repository->findBy(['fieldName' => [null]);
$repository->findBy(['fieldName' => [123, null]);
```

Where before, `null` was just ignored