Filter out empty and duplicate values in Comment Moderation and Comment Blacklist settings.

props GaVrA.
fixes #23800.
Built from https://develop.svn.wordpress.org/trunk@28444


git-svn-id: http://core.svn.wordpress.org/trunk@28271 1a063a9b-81f0-0310-95a4-ce76da25c4cd