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

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Variations of this test already exist:
ecl/regress/regroup3.ecl
testing/regress/ecl/combinelocal.ecl
testing/regress/ecl/combine.ecl