Fix revisions title for pages without title with useheading on FS#2597

inc/html.php

@@ -464,6 +464,7 @@ function html_revisions($first=0, $media_id = false){
     else $exists = @file_exists(mediaFN($id));
 
     $display_name = (!$media_id && useHeading('navigation')) ? hsc(p_get_first_heading($id)) : $id;
+    if (!$display_name) $display_name = $id;
 
     if($exists && $first==0){
         if (!$media_id && isset($INFO['meta']) && isset($INFO['meta']['last_change']) && $INFO['meta']['last_change']['type']===DOKU_CHANGE_TYPE_MINOR_EDIT)