[WIP] Datagrid creation raise an error when collection is empty

When trying to create a datagrid with an empty collection, the following error is raised :

> Argument 1 passed to Jfsimon\Datagrid\Infra\Extension\CoreExtension::guessSchema() must be an instance of Jfsimon\Datagrid\Model\Data\Entity, null given, called in Jfsimon/Datagrid/Infra/Factory/Factory.php on line 90 and defined in Jfsimon/Datagrid/Infra/Extension/CoreExtension.php line 33