Fix folder name extraction functions

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

src/base/utils/fs.cpp

@@ -95,7 +95,7 @@ QString Utils::Fs::folderName(const QString &filePath)
     const QString path = toUniformPath(filePath);
     const int slashIndex = path.lastIndexOf('/');
     if (slashIndex == -1)
-        return path;
+        return {};
     return path.left(slashIndex);
 }
 

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);
     };