[SPARK-2327] [SQL] Fix nullabilities of Join/Generate/Aggregate.

Fix nullabilities of `Join`/`Generate`/`Aggregate` because:
- Output attributes of opposite side of `OuterJoin` should be nullable.
- Output attributes of generater side of `Generate` should be nullable if `join` is `true` and `outer` is `true`.
- `AttributeReference` of `computedAggregates` of `Aggregate` should be the same as `aggregateExpression`'s.

Existing: add `override` when implementing abstract methods.