HPCC-11399 Fix request with JSON nulls being treated as empty not null

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5937 from afishbeck/jsonNullFieldsNotDefaulting

HPCC-11399 Fix request with JSON nulls being treated as empty not null

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>