HPCC-11234 Roxie gives UNIMPLEMENTED if try to read result from workunit

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5704 from richardkchapman/roxie-wu-read

HPCC-11234 Roxie gives UNIMPLEMENTED if try to read result from workunit

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>