Only show the column preferences if we have columns. See #13393 props duck_.

git-svn-id: http://svn.automattic.com/wordpress/trunk@14635 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/template.php

@@ -3610,7 +3610,7 @@ function screen_meta($screen) {
 			<br class="clear" />
 		</div>
 		<?php endif;
-		if ( isset($column_screens) ) : ?>
+		if ( ! empty($column_screens) ) : ?>
 		<h5><?php echo apply_filters('columns_prefs_header', $default_text); ?></h5>
 		<div class="metabox-prefs">
 			<?php manage_columns_prefs($screen); ?>