[SPARK-10847][SQL][PYSPARK] Pyspark - DataFrame - Optional Metadata with...

[SPARK-10847][SQL][PYSPARK] Pyspark - DataFrame - Optional Metadata with `None` triggers cryptic failure

The error message is now changed from "Do not support type class scala.Tuple2." to "Do not support type class org.json4s.JsonAST$JNull$" to be more informative about what is not supported. Also, StructType metadata now handles JNull correctly, i.e., {'a': None}. test_metadata_null is added to tests.py to show the fix works.

Author: Jason Lee <cjlee@us.ibm.com>

Closes #8969 from jasoncl/SPARK-10847.