Filemanager download all doesn't download anything if hierarchical folder is selected and doesn't have any file.

Steps to reproduce:

1. Log in as admin
2. Go to "My private files"
3. Change view to tree view
4. Create folder (abc) and don't add any file.
5. Select folder (abc) and click "create folder" (xyz)
6. Save changes
7. change view to "Tree view"
8. Select folder (xyz) and click "Download all"

Actual output:
Nothing happens

Expected output:
pop-up with saving zip option / warning saying no files to download / "Download all" option should be hidden.