Fix folder name extraction functions

It should return empty string if there is no parent folder.

src/webui/www/private/scripts/filesystem.js

@@ -70,7 +70,7 @@ window.qBittorrent.Filesystem = (function() {
     const folderName = function(filepath) {
         const slashIndex = filepath.lastIndexOf(PathSeparator);
         if (slashIndex === -1)
-            return filepath;
+            return '';
         return filepath.substring(0, slashIndex);
     };