panic: GROUP BY or SELECT DISTINCT over a zero-field (empty) struct key

[fornwall]

Describe the bug

GROUP BY (and SELECT DISTINCT) over a group key whose type contains a zero-field struct (DataType::Struct(Fields::empty())) panics when the aggregate emits.

A zero-field struct array has no child to derive its length from, so arrow-rs refuses StructArray::try_new for it. The group-value emit path rebuilds the key column with that constructor and unwraps the resulting error.

To Reproduce

$ datafusion-cli -c "select arrow_cast(null, 'Struct()') as s, count(*) from (values (1),(2),(3)) group by 1;"
DataFusion CLI v55.1.0

thread 'tokio-rt-worker' panicked at datafusion/physical-plan/src/aggregates/group_values/multi_group_by/row_backed.rs:225:14:
dictionary re-encode during emit: ArrowError(InvalidArgumentError("use StructArray::try_new_with_length or StructArray::new_empty_fields to create a struct array with no fields so that the length can be set correctly"), Some(""))
Error: Join Error
caused by
External error: task 63 panicked with message "dictionary re-encode during emit: ..."

Same panic for SELECT DISTINCT:

select distinct arrow_cast(null, 'Struct()') as s from (values (1),(2));

and when the zero-field struct is nested inside the key rather than being the whole key:

select named_struct('a', column1, 'z', arrow_cast(null, 'Struct()')) as s, count(*)
from (values (1),(2)) group by 1;

Expected behavior

The query returns its groups. A zero-field struct is a valid Arrow type and carries one bit per row (null or not), so it should group like any other key.

Additional context

Reproduced on main at 15f32dd.