BUG #19621: Unexpected results of JSON_VALUE with DEFAULT ON EMPTY

The following bug has been logged on the website:

Bug reference: 19621
Logged by: Suyang Zhong
Email address: syzhong16(at)gmail(dot)com
PostgreSQL version: 19beta3
Operating system: Ubuntu 22.04
Description:

Consider the following test case:

```sql
CREATE TABLE t0(x text);
INSERT INTO t0 VALUES ('{}'), (NULL);
CREATE VIEW v0 AS
SELECT x, json_value(x, '$.a' RETURNING int DEFAULT 42 ON EMPTY) AS jv
FROM t0;

SELECT x IS NULL AS is_null, jv FROM v0;
-- f | 42
-- t | 42

SELECT jv FROM v0 WHERE x IS NULL;
-- NULL
```

The second query selects exactly the row the first query shows as `t | 42`,
yet returns NULL for its `jv`.

A further reduction shows that the result depends on the input row order.

```sql
SELECT json_value(x, '$.a' RETURNING int DEFAULT 42 ON EMPTY) FROM (VALUES
('{}'), (NULL)) v(x);
-- 42
-- 42
SELECT json_value(x, '$.a' RETURNING int DEFAULT 42 ON EMPTY) FROM (VALUES
(NULL), ('{}')) v(x);
-- NULL
-- 42
```

Reproduced on 20devel, and on 17.11, 18.1, 18.4, and 19beta3; on 17rc1 only
the `ON ERROR` variants misbehave.