sql: UDF returning SETOF VOID should produce zero rows

In Postgres:

```
marcus=# create function f() returns setof void language sql as 'select null';
CREATE FUNCTION

marcus=# select f();
f
---
(0 rows)
```

In CRDB:

```
defaultdb> create function f() returns setof void language sql as 'select null';
CREATE FUNCTION

defaultdb> select f();
f
--------
NULL
(1 row)
```

I'm actually skeptical of PG's behavior here, and I'm not convinced we want to mimic it. It seems like `SELECT ` should always return a single row, and I don't have an explanation for why it doesn't in this case.

Jira issue: CRDB-27839