Uploading Files to Subdirectories on seafile backends doesn't work

Summary:
This patch fixes uploading for new files. Tested with Seafile 8.0 and 9.0.
It also removes some php warnings when fileapi_backend_storage_disabled is
disabled

Test Plan:
Connect a Seafile storage. Create a SubDirectory and try to upload
a file.

Reviewers: #chwala_developers, machniak, mollekopf

Reviewed By: #chwala_developers, mollekopf

Tags: #chwala

Differential Revision: https://git.kolab.org/D3475