Fixes in admin-bar.js:

- Silence errors when a node doesn't exist similarly to jQuery.
- Add "feature testing" and fallbacks for old browsers as this may run on the front-end.
- Improve inline docs.

Props dinhtungdu, azaozz.
Merges [46883] to the 5.3 branch.
Fixes #47069.



git-svn-id: https://develop.svn.wordpress.org/branches/5.3@46884 602fd350-edb4-49c9-b593-d223f7449a82