Menus: Avoid a PHP warning in `add_submenu_page()` when the same value is...

Menus: Avoid a PHP warning in `add_submenu_page()` when the same value is passed for both `$parent_slug` and `$menu_slug` parameters.

Props welcher, ispreview, ayeshrajans, NextScripts, adamsilverstein, garrett-eclipse, 123host.
Fixes #48599.
Built from https://develop.svn.wordpress.org/trunk@46868


git-svn-id: https://core.svn.wordpress.org/trunk@46668 1a063a9b-81f0-0310-95a4-ce76da25c4cd