feat: fix AVG sliding windows wrong results with NULLs

[comphead]

Which issue does this PR close?

• Closes Windows AVG wrong result for NULLs #22138 .

Rationale for this change

AVG used as a window aggregate can return NaN (and, for Decimal / Duration, panic on integer division by zero) when every value in the window frame is NULL.

SELECT i,
       AVG(v) OVER (ORDER BY i ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING)
FROM (VALUES (1,1), (2,2), (3,NULL), (4,NULL)) t(i,v);

i	current output	expected (DuckDB/PgSQL)
1	1.5	1.5
2	2.0	2.0
3	NaN	NULL
4	NaN	NULL

Root cause: sliding-window execution calls Accumulator::retract_batch as rows leave the frame. Once every contributing value has been retracted, self.count drops back to 0 but self.sum stays
Some(0.0) (or a tiny floating-point residual). evaluate() then computes sum / 0, which yields NaN on Float64, and would panic with integer division by zero on DecimalAvgAccumulator and
DurationAvgAccumulator.

The non-sliding aggregation path is unaffected because there sum becomes Some(_) only after at least one non-NULL value has been added, so count == 0 implies sum == None.

What changes are included in this PR?

datafusion/functions-aggregate/src/average.rs — guard all three affected evaluate() implementations with an explicit count == 0 → None short-circuit:

• AvgAccumulator::evaluate (Float64)
• DecimalAvgAccumulator::evaluate (Decimal32/64/128/256)
• DurationAvgAccumulator::evaluate (Duration*)

This matches the idiom already used by sibling retractable accumulators (variance.rs uses an explicit match self.count before division; sum.rs uses a (self.count != 0).then_some(..) guard).

[2010YOUY01]

LGTM, thank you!