HPCC-9438 Fix problem with unusual joins against null datasets.

Regression introduced by HPCC-9239 (pull #4405)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4463 from ghalliday/issue9438

HPCC-9438 Fix problem with unusual joins against null datasets.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>