Fix possible wrong return value of Folder::getFolderParent
Fix possible wrong return value of Folder::getFolderParent

Summary:
The method `Folder::getFolderParent` must only return
objects of type `FolderInterface` or `null` if no parent could be found.

Reviewers: #forgetit-dev, dkd-ebert

Reviewed By: #forgetit-dev, dkd-ebert

Differential Revision: https://phabricator.dkd.de/D806

dkd/php-cmis-client