Point lookup index should support is null/is not null

for secondary index, need to support is null/not null predicate

select * from mytable where secCol is null;

today this does not trigger index look up

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-9549
- Type: Bug
- Affects version(s):
- 1.2.0

Start by reproducing the reported query, `select * from mytable where secCol is null`, and trace the secondary-index point-lookup path that handles predicates. Compare it with `is not null` and verify that both predicates trigger the index lookup; done means the lookup is used for these cases without changing other query behavior.

Written by the indexing model from the issue text.