[SPARK-1926] [SQL] Nullability of Max/Min/First should be true.

Nullability of `Max`/`Min`/`First` should be `true` because they return `null` if there are no rows.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #881 from ueshin/issues/SPARK-1926 and squashes the following commits:

322610f [Takuya UESHIN] Fix nullability of Min/Max/First.

(cherry picked from commit d1375a2bff846f2c4274e14545924646852895f9)
Signed-off-by: Reynold Xin <rxin@apache.org>