Native NULL Support requires explicitly filtering the records

While Pinot does support storing NULLs natively, users are still expected to filter them out in the `where clause` when querying. It would be great to have the support for following:

- Filter out nulls using `CASE WHEN`, for example `sum(case when description is null then 1 else 0 end)`

No files, tests, or entry points are identified. Start by tracing Pinot's SQL expression and aggregation handling for CASE WHEN with IS NULL, then locate existing NULL-semantics tests. Done means the example query executes and counts NULL values without requiring a WHERE filter, with regression coverage.

Written by the indexing model from the issue text.