[SPARK-1819] [SQL] Fix GetField.nullable.

`GetField.nullable` should be `true` not only when `field.nullable` is `true` but also when `child.nullable` is `true`.

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

Closes #757 from ueshin/issues/SPARK-1819 and squashes the following commits:

8781a11 [Takuya UESHIN] Modify a test to use named parameters.
5bfc77d [Takuya UESHIN] Fix GetField.nullable.