[SPARK-15441][SQL] support null object in Dataset outer-join

## What changes were proposed in this pull request?

Currently we can't encode top level null object into internal row, as Spark SQL doesn't allow row to be null, only its columns can be null.

This is not a problem before, as we assume the input object is never null. However, for outer join, we do need the semantics of null object.

This PR fixes this problem by making both join sides produce a single column, i.e. nest the logical plan output(by `CreateStruct`), so that we have an extra level to represent top level null obejct.

## How was this patch tested?

new test in `DatasetSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13425 from cloud-fan/outer-join2.