[SPARK-2875] [PySpark] [SQL] handle null in schemaRDD()

Handle null in schemaRDD during converting them into Python.

Author: Davies Liu <davies.liu@gmail.com>

Closes #1802 from davies/json and squashes the following commits:

88e6b1f [Davies Liu] handle null in schemaRDD()

(cherry picked from commit 48789117c2dd6d38e0bd8d21cdbcb989913205a6)
Signed-off-by: Michael Armbrust <michael@databricks.com>