Add NULL support in count function

To support queries like `count(col)` where `col` can have NULL values, or `count(CASE WHEN condition THEN v END)`

In certain cases, there's no workaround to calculating such values.

The issue concerns Pinot query handling for count(col) and count(CASE WHEN condition THEN v END) when values are NULL. Start by tracing the count-function implementation and its existing tests; done means these expressions return the expected counts without requiring a workaround.

Written by the indexing model from the issue text.