plpgsql: unnamed routine parameters are not handled correctly

There's a pg_regress test failing even after the fix in #143887:
```
root@localhost:26257/defaultdb> create function tftest(int) returns table(a int, b int) as $$
-> begin
-> return query select $1, $1+i from generate_series(1,5) g(i);
-> end;
-> $$ language plpgsql immutable strict;
-> select * from tftest(10);
CREATE FUNCTION

Time: 59ms total (execution 30ms / network 29ms)

a | b
-------+-------
NULL | NULL
NULL | NULL
NULL | NULL
NULL | NULL
NULL | NULL
(5 rows)

Time: 1ms total (execution 1ms / network 0ms)
```

Jira issue: CRDB-49883