HPCC-9265 - lightweight selfjoin hint(parallel_match) issue

A bug in the selfjoin helper was mishandling lightweight
selfjoin, with the reslut that it thought there was no RHS
resulting in zero matches for a standard inner join.
Similar issues if limited substring match join used.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge branch 'hpcc-9265' into candidate-4.0.0

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>