Filtered aggregation fails when a group has no matching rows

[lyne7-sc]

Describe the bug

A grouped aggregate with a FILTER clause can fail when a group exists but none of its rows satisfy the aggregate filter.

The grouping key is created independently of the aggregate filter. However, the accumulator is currently expanded only when aggregate rows are updated. If all rows for an aggregate are filtered out, its accumulator may remain shorter than the number of groups.

The query then fails while producing the result:

index out of bounds: the len is 0 but the index is 0

To Reproduce

Run a filtered aggregation where one merchant has no refunded orders:

WITH orders AS (
  SELECT * FROM VALUES
    ('A', 100, 'PAID'),
    ('A',  20, 'REFUNDED'),
    ('B', 200, 'PAID'),
    ('B', 300, 'PAID')
  AS orders(merchant_id, amount, status)
)
SELECT
  merchant_id,
  SUM(amount) FILTER (WHERE status = 'PAID') AS paid_amount,
  COUNT(*) FILTER (WHERE status = 'REFUNDED') AS refund_count
FROM orders
GROUP BY merchant_id
ORDER BY merchant_id;

Expected behavior

All discovered groups should be preserved, using each aggregate's empty-input value:

A | 100 | 1
B | 500 | 0

Screenshots

Additional context