Fallback to share link owner when no owner found

When creating link shares from external storage, the filesystem cannot
find an owner in some scenarios (ex: system-wide mounts). In such
cases, fall back to using the current user's trashbin which happens to
also be the user who created the link share.

Fixes an issue where this scenario made deletion impossible due to
missing user information.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>