JMenu: Codestyle

- updated comments in docblock
- `$menu->getDefault()` now returns null in case of failure like other menu methods. This shouldn't affect anything, as return is evaluated using `is_object` or not at all.
  - [JPatchwaySite](https://github.com/piotr-cz/joomla-cms/blob/master/libraries/cms/pathway/site.php#L40)
  - [JRouterSite](https://github.com/piotr-cz/joomla-cms/blob/master/libraries/cms/router/site.php#L155)