Fixed passing current_folder to DBus for save file dialog (#13491)

Co-authored-by: Herman Kirshin <herman.kirshin@jetbrains.com>

src/Avalonia.FreeDesktop/DBusSystemDialog.cs

@@ -88,7 +88,7 @@ namespace Avalonia.FreeDesktop
             if (options.SuggestedFileName is { } currentName)
                 chooserOptions.Add("current_name", new DBusVariantItem("s", new DBusStringItem(currentName)));
             if (options.SuggestedStartLocation?.TryGetLocalPath()  is { } folderPath)
-                chooserOptions.Add("current_folder", new DBusVariantItem("ay", new DBusByteArrayItem(Encoding.UTF8.GetBytes(folderPath))));
+                chooserOptions.Add("current_folder", new DBusVariantItem("ay", new DBusByteArrayItem(Encoding.UTF8.GetBytes(folderPath + "\0"))));
 
             objectPath = await _fileChooser.SaveFileAsync(parentWindow, options.Title ?? string.Empty, chooserOptions);
             var request = new OrgFreedesktopPortalRequest(_connection, "org.freedesktop.portal.Desktop", objectPath);