Bugfix Format::forge()->to_(json|serialized|php)

Loose typing provokes nullish values to be converted to null. Hence encoding empty array() doesn't work.

Strangely enough to_yaml() behaviour doesn't seem to be affected. But both `Format::forge(array())->to_json()` and `Format::forge()->to_yaml(array())` returns `---\n` (triple dash followed by a line break)

Before the patch :

    Format::forge(array())->to_json() === '[]';
    Format::forge()->to_json(array()) === 'null';

    Format::forge(array())->to_serialized() === 'a:0:{}';
    Format::forge()->to_serialized(array()) === 'N;';*

    Format::forge(array())->to_php() === "array (\n)";
    Format::forge()->to_serialized(array()) === 'NULL';

After the patch `Format::forge()->to_anything(array())` returns the same as `Format::forge(array())->to_anything()`