[SPARK-2504][SQL] Fix nullability of Substring expression.

This is a follow-up of #1359 with nullability narrowing.

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

Closes #1426 from ueshin/issues/SPARK-2504 and squashes the following commits:

5157832 [Takuya UESHIN] Remove unnecessary white spaces.
80958ac [Takuya UESHIN] Fix nullability of Substring expression.