HPCC-8413 Stepped join don't split results of previous graph iterations

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3889 from ghalliday/issue8413b

HPCC-8413 Stepped join don't split results of previous graph iterations

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>