[SPARK-16097][SQL] Encoders.tuple should handle null object correctly

## What changes were proposed in this pull request?

Although the top level input object can not be null, but when we use `Encoders.tuple` to combine 2 encoders, their input objects are not top level anymore and can be null. We should handle this case.

## How was this patch tested?

new test in DatasetSuite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13807 from cloud-fan/bug.