HPCC-8516 Fix problem with ROWS(RIGHT) in keyed denormalize

If the index used in a keyed denormalize(,group) contained a child dataset
then a reference to ROWS(RIGHT) would generate an internal error.  This is
because it was acccessing a deserialize dataet, but the dataset passed in
is serialized.
Merge pull request #3823 from ghalliday/issue8516

HPCC-8516 Fix problem with ROWS(RIGHT) in keyed denormalize

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>