Media Modal: Check for empty keys when saving attachment captions. see #21390, #21807.

git-svn-id: http://core.svn.wordpress.org/trunk@22200 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/ajax-actions.php

@@ -1839,10 +1839,10 @@ function wp_ajax_save_attachment() {
 	$changes = $_REQUEST['changes'];
 	$args    = array();
 
-	if ( $changes['title'] )
+	if ( ! empty( $changes['title'] ) )
 		$args['post_title'] = $changes['title'];
 
-	if ( $changes['caption'] )
+	if ( ! empty( $changes['caption'] ) )
 		$args['post_excerpt'] = $changes['caption'];
 
 	if ( $args )