In `media-editor.js`, use `_.isUndefined()` when available.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27317 602fd350-edb4-49c9-b593-d223f7449a82
Remove debugging cruft, see #25663

git-svn-id: https://develop.svn.wordpress.org/trunk@27318 602fd350-edb4-49c9-b593-d223f7449a82