[SPARK-12478][SQL] Bugfix: Dataset fields of product types can't be null

When creating extractors for product types (i.e. case classes and tuples), a null check is missing, thus we always assume input product values are non-null.

This PR adds a null check in the extractor expression for product types. The null check is stripped off for top level product fields, which are mapped to the outermost `Row`s, since they can't be null.

Thanks cloud-fan for helping investigating this issue!

Author: Cheng Lian <lian@databricks.com>

Closes #10431 from liancheng/spark-12478.top-level-null-field.