[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.