Merge pull request #3887 from ghalliday/issue8413

HPCC-8413 Fix problems with STEPPED JOIN accessing non existant results

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>
HPCC-8413 Stepped join don't split results of previous graph iterations

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>