Support StarTree index with nullHandlingEnabled

Not sure if this is technically feasible but would be great to have this support. Perhaps during ST index creation, the null vector index could be used to skip adding specific values to the startree index?

Start by investigating Pinot's StarTree index creation and how nullHandlingEnabled is currently handled. Determine whether null values can be excluded from the StarTree index without breaking existing behavior, then define and verify the expected support with tests; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.