WP_Editor: don't show empty div if no editor-buttons and media-buttons are...

WP_Editor: don't show empty div if no editor-buttons and media-buttons are shown, props ocean90, see #17144

git-svn-id: https://core.svn.wordpress.org/trunk@18519 1a063a9b-81f0-0310-95a4-ce76da25c4cd