[Gapfill] NPE when a column is included in `TIMESERIESON` but not selected

[Jackie-Jiang]

Query example:
SELECT GAPFILL(DATETIMECONVERT(dt, ...), '', '', '', FILL(requests, 'FILL_DEFAULT_VALUE'), TIMESERIESON(publisher_id)) as gdt, requests

Exception is currently thrown from:
org.apache.pinot.core.query.reduce.GapfillProcessor.process(:88)

We may decide whether to support this use case. If we don't want to support this right now, we should add some validation for the GAPFILL and throw BadRequestException when a query cannot be processed.