Avoid notice when themes dir is empty or missing. Don't reset current theme...

Avoid notice when themes dir is empty or missing.  Don't reset current theme in current_theme_info() if there are no themes.  Show warning in Right Now if there are no themes.  Props azaozz. fixes #19089

git-svn-id: https://core.svn.wordpress.org/trunk@19251 1a063a9b-81f0-0310-95a4-ce76da25c4cd