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

See #26631.


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Remove debugging cruft, see #25663
Built from https://develop.svn.wordpress.org/trunk@27318


git-svn-id: http://core.svn.wordpress.org/trunk@27170 1a063a9b-81f0-0310-95a4-ce76da25c4cd