HPCC-8486 Optimize DICTIONARY(null-dataset)

Also adds support for EXISTS(dictionary) and optimizes the null case.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #3986 from richardkchapman/dict-null

HPCC-8486 Optimize DICTIONARY(null-dataset)

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>