When copying new bundled themes & plugins, bail early if the distro doesn't include the bundled item. This prevents us from creating an empty directory in the destination when the source doesn't exist. See #22856. Merges [23179] to the 3.5 branch


git-svn-id: https://core.svn.wordpress.org/branches/3.5@23251 1a063a9b-81f0-0310-95a4-ce76da25c4cd