Because the REST API allows meta keys to have empty values, the Custom Fields meta box should permit the same behavior. Props charlestonsw, soulseekah, danielbachhuber. Merges [43811] to trunk. Fixes #43559. git-svn-id: https://develop.svn.wordpress.org/trunk@44153 602fd350-edb4-49c9-b593-d223f7449a82