uptobox: fix rmdir declaring that directories weren't empty

The API seems to have changed and the `totalFileCount` item no longer
tracks the number of files in the directory so is useless for seeing
if the directory is empty.

This patch fixes the problem by seeing whether there are any files or
directories in the folder instead.

This problem was detected by the integration tests.

backend/uptobox/uptobox.go

@@ -678,7 +678,7 @@ func (f *Fs) Rmdir(ctx context.Context, dir string) error {
 	if err != nil {
 		return err
 	}
-	if info.Data.CurrentFolder.FileCount > 0 {
+	if len(info.Data.Folders) > 0 || len(info.Data.Files) > 0 {
 		return fs.ErrorDirectoryNotEmpty
 	}