Cast $wp_theme_directories to array. This had been done in get_themes(). A...

Cast $wp_theme_directories to array. This had been done in get_themes(). A must-loaded plugin could call these functions before the first theme root is ever added. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20116 1a063a9b-81f0-0310-95a4-ce76da25c4cd