sftp: check directory is empty before issuing rmdir
sftp: check directory is empty before issuing rmdir

Some SFTP servers allow rmdir on full directories which is allowed
according to the RFC so make sure we don't accidentally delete data
here.

See: https://forum.rclone.org/t/rmdir-and-delete-empty-src-dirs-file-does-not-exist/7737

olihey/rclone,B4dM4n/rclone,B4dM4n/rclone,olihey/rclone,olihey/rclone,ncw/rclone,B4dM4n/rclone,ncw/rclone,ncw/rclone,ncw/rclone,olihey/rclone,B4dM4n/rclone