allow NULL appNamespace for resource loader again

If an application would like to use a appNamespace of NULL (but not an empty string!) the resource loader should load resources without prefixing them. For example Form_Foo from /application/forms/Foo.php. This behaviour is optional as the default value and still usable option to disable the resource loader is setting the appNamespace to FALSE.

http://framework.zend.com/issues/browse/ZF-9435