Remove NULL handling in Type::cast()

[thekid]

This pull request changes the special handling in Type::Cast() which will return null when null is given, and changes it to work like the cast operators in PHP. The old behavior can be achieved with nullable types.

Examples

Primitives:

use lang\Primitive;

// Both now return an empty string
$r= (string)null; 
$r= Primitve::$STRING->cast(null);

Arrays:

use lang\Type;

// Both now return an empty array
$r= (array)null;
$r= Type::$ARRAY->cast(null);

Objects:

use lang\Type;

// Both now return an empty object
$r= (object)null;
$r= Type::$OBJECT->cast(null);