[SPARK-2509][SQL] Add optimization for Substring.

`Substring` including `null` literal cases could be added to `NullPropagation`.

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

Closes #1428 from ueshin/issues/SPARK-2509 and squashes the following commits:

d9eb85f [Takuya UESHIN] Add Substring cases to NullPropagation.

(cherry picked from commit 9b38b7c71352bb5e6d359515111ad9ca33299127)
Signed-off-by: Reynold Xin <rxin@apache.org>