Fix cancellation of UploadFolder dialogs

Remove the extra "/" element from the list of results and pass
an empty file list to the backend.

Task-number: QTBUG-61186
Change-Id: Ib2c577c15502e8d1506948e9683921ed211b9c48
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>