Toolbar: Prevent PHP deprecation warning in admin bar when node is added with a `null` parent.

Developed in https://github.com/WordPress/wordpress-develop/pull/11055

Props sky_76, westonruter, ozgursar, sabernhardt.
Fixes #64728.


git-svn-id: https://develop.svn.wordpress.org/trunk@61997 602fd350-edb4-49c9-b593-d223f7449a82