HPCC-9592 Generate a simpler graph for IF(cond, <EMPTY>, ds)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4578 from ghalliday/issue9592

HPCC-9592 Generate a simpler graph for IF(cond, <EMPTY>, ds)

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>