[SPARK-2965][SQL] Fix HashOuterJoin output nullabilities.

Output attributes of opposite side of `OuterJoin` should be nullable.

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

Closes #1887 from ueshin/issues/SPARK-2965 and squashes the following commits:

bcb2d37 [Takuya UESHIN] Fix HashOuterJoin output nullabilities.