[SPARK-2535][SQL] Add StringComparison case to NullPropagation.

`StringComparison` expressions including `null` literal cases could be added to `NullPropagation`.

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

Closes #1451 from ueshin/issues/SPARK-2535 and squashes the following commits:

e99c237 [Takuya UESHIN] Add some tests.
8f9b984 [Takuya UESHIN] Add StringComparison case to NullPropagation.