Upgrade: When upgrading from WordPress 3.5, if an empty twentytwelve theme...

Upgrade: When upgrading from WordPress 3.5, if an empty twentytwelve theme directory exists, remove it to allow the installation of Twenty Twelve. See #22856. For trunk


git-svn-id: https://core.svn.wordpress.org/trunk@23253 1a063a9b-81f0-0310-95a4-ce76da25c4cd