string_agg(..., NULL) uses a comma in DataFusion AggregateScan

CREATE TABLE string_agg_products (
    id integer PRIMARY KEY,
    description text NOT NULL
);

CREATE TABLE string_agg_tags (
    id integer PRIMARY KEY,
    product_id integer NOT NULL,
    tag_name text NOT NULL
);

INSERT INTO string_agg_products
VALUES (1, 'Laptop with fast processor');

INSERT INTO string_agg_tags
VALUES (1, 1, 'tech'), (2, 1, 'tech');

CREATE INDEX string_agg_products_idx
ON string_agg_products USING paradedb (id, description);

CREATE INDEX string_agg_tags_idx
ON string_agg_tags USING paradedb (id, product_id, tag_name)
WITH (
    text_fields = '{"tag_name":{"fast":true}}'
);

SET paradedb.enable_aggregate_custom_scan = off;

SELECT string_agg(t.tag_name, NULL)
FROM string_agg_products p
JOIN string_agg_tags t ON p.id = t.product_id
WHERE p.description @@@ 'laptop';
-- techtech

SET paradedb.enable_aggregate_custom_scan = on;

EXPLAIN (COSTS OFF)
SELECT string_agg(t.tag_name, NULL)
FROM string_agg_products p
JOIN string_agg_tags t ON p.id = t.product_id
WHERE p.description @@@ 'laptop';

SELECT string_agg(t.tag_name, NULL)
FROM string_agg_products p
JOIN string_agg_tags t ON p.id = t.product_id
WHERE p.description @@@ 'laptop';
-- tech,tech