HPCC-12490 Add getGraphsMeta() to explicitly get the meta info

Fix regression introduced by 1st PR (gh-6592)
A workunit without any progress caused a crash.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6636 from jakesmith/hpcc-12490-2

HPCC-12490 Add getGraphsMeta() to explicitly get the meta info

Reviewed-By: Kevin Wang <kevin.wang@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>