Page as a child menu item return null to parent() method

- Corcel Version: 2.5
- Framework Name & Version: Laravel 5.4
- PHP Version: 7.1
- Database Driver & Version: MySQL Ver 14.14 Distrib 5.7.17

### Description:

When the menu item is a child and a page at the same time, the parent is null.

### Steps To Reproduce:

Create a menu in your backoffice with a page as a root item child. Iterate this menu in laravel or blade or PHP. The $item->parent() or $item->first()->parent() returns null.