…4582)

# Ticket(s) Closed

- Closes #

## What

- `GROUP BY` on a nullable bool column with the aggregate custom scan
crashed with: `InvalidArgument("Missing value U64(2) for field ... is
not supported for column type Bool")`
- The cause was that Tantivy's terms aggregation rejects all numeric
`Key` variants for Bool columns
- Instead, use `Key::Str(NULL_SENTINEL_*)` for Bool fields, which routes
through Tantivy's `MissingTermAgg` collector

## Why

## How

## Tests

Added regression test

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Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
(cherry picked from commit 99b6ddf)