plpgsql: support RECORD-returning UDFs that don't have concrete type RETURN in the root block

We currently don't support cases when RECORD-returning UDF doesn't have a RETURN stmt in the root block that would restrict the wildcard type to a concrete one. For example:
```sql
CREATE FUNCTION f1() RETURNS RECORD AS $$
BEGIN
END;
$$ LANGUAGE PLpgSQL;

CREATE FUNCTION f2(b BOOL) RETURNS RECORD AS $$
BEGIN
IF b THEN
RETURN f1();
ELSIF true THEN
DECLARE
BEGIN
END;
END IF;
END;
$$ LANGUAGE PLpgSQL;
```

Jira issue: CRDB-38131