sub-select fails to run `json_object_agg` safely

**Describe the problem**

When run as a projected single-column sub-select, `json_object_agg()` appears to fail when there are no rows being aggregated on, but only when the filtering condition uses columns from the correlated outer query.

The query:

```
select
(
select
-- count(y) <- this works, and returns [0, 1]
json_object_agg('how could this', 'possibly fail?')

from unnest('{0,1}'::text[]) as u(y)
where
t.x = u.y
)
from unnest('{1,2}'::text[]) as t(x)
```

fails with the error:

```
ERROR: field name must not be null
```

For comparison, when using instead `where false` the result is two rows of `NULL`, as expected.

**To Reproduce**

The query is self-contained. Simply run it with `psql` or similar.

**Expected behavior**

Postgres produces this output for the query:

```
json_object_agg
-----------------------------------------

{ "how could this" : "possibly fail?" }
(2 rows)
```

**Additional data / screenshots**

**Environment:**
- CockroachDB version v22.2.8 (via docker image)
- Server OS: Linux Nixos
- Client app : psql

**Additional context**
What was the impact?

Add any other context about the problem here.

Jira issue: CRDB-31121