Fix top query regression tests on PG 12,13

On PG 12, 13, the internal return instruction in the following function:
```
CREATE OR REPLACE FUNCTION add(int, int) RETURNS INTEGER AS
  $$
  BEGIN
     return (select $1 + $2);
  END; $$ language plpgsql;
```

Is stored as SELECT (select expr1 + expr2)

On PG 14 onward it's stored just as SELECT (expr1 + expr2)

regression/expected/top_query_1.out

@@ -37,7 +37,7 @@ SELECT query, top_query FROM pg_stat_monitor ORDER BY query COLLATE "C";
          return add($1,$2);                                  +| 
  END;                                                        +| 
  $$ language plpgsql                                          | 
- SELECT (select $1 + $2)                                      | SELECT add2(1,2);
+ SELECT (select NULL + NULL)                                  | SELECT add2(1,2);
  SELECT add2(1,2)                                             | 
  SELECT pg_stat_monitor_reset()                               | 
 (5 rows)