HPCC-12491 Fix core when COMBINE doesn't have a transform

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6693 from ghalliday/issue12491

HPCC-12491 Fix core when COMBINE doesn't have a transform

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>