feat: client->getFolderPath return null if folder is not set. (Webklex#506)

* feat: client->getFolderPath return null if folder is not set.

* docs: update comment

Author: arnolem

src/Client.php

@@ -773,9 +773,9 @@ public function checkFolder(string $folder_path): array {
     /**
      * Get the current active folder
      *
-     * @return string
+     * @return null|string
      */
-    public function getFolderPath(): string {
+    public function getFolderPath(): ?string {
         return $this->active_folder;
     }