[SPARK-11980][SPARK-10621][SQL] Fix json_tuple and add test cases for

Added Python test cases for the function `isnan`, `isnull`, `nanvl` and `json_tuple`.

Fixed a bug in the function `json_tuple`

rxin , could you help me review my changes? Please let me know anything is missing.

Thank you! Have a good Thanksgiving day!

Author: gatorsmile <gatorsmile@gmail.com>

Closes #9977 from gatorsmile/json_tuple.