HPCC-11053 - Fix regression if keyedjoin fetch file is missing

If using ,OPT on the dataset of an index, there was a
serialization error. Regression introduced by HPCC-10664

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5572 from jakesmith/hpcc-11053

HPCC-11053 - Fix regression if keyedjoin fetch file is missing

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>