AssetAdmin: PHP warning trying to add empty values into CompositeField

If the logged in user doesn't have permission to add a Folder record,
AssetAdmin::getEditForm() tries to enter an empty value into the
children of a ComposteField. This breaks SSViewer with a
call_user_func invalid args PHP warning.

Only include these buttons when they're available to the user.

Affects SilverStripe 3.0 and 3.1.