[SPARK-2968][SQL] Fix nullabilities of Explode.

Output nullabilities of `Explode` could be detemined by `ArrayType.containsNull` or `MapType.valueContainsNull`.

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

Closes #1888 from ueshin/issues/SPARK-2968 and squashes the following commits:

d128c95 [Takuya UESHIN] Fix nullability of Explode.

(cherry picked from commit c686b7dd4668b5e9fc3177f15edeae3446d2e634)
Signed-off-by: Michael Armbrust <michael@databricks.com>