LEAD/LAG ignores default value

LEAD and LAG functions ignore their 3rd argument  (default value) and return `null` regardless its value:
```
  SELECT
    FLOOR(__time TO DAY) t,
    cnt,
    LEAD(cnt) OVER (),
    LEAD(cnt,1,999) OVER ()
  FROM foo
```
the resultset is
```
  - [946684800000,1,null,null]
 [...]
```
