[SPARK-1630] Turn Null of Java/Scala into None of Python

During serializing PythonRDD, it will cause an NPE if there null in it. This patch will handle it as None of Python.

This PR is based on #554, thanks to @kalpit. 
This method isn't tail-recursive, so this will cause a StackOverflow if you try to write an iterator with thousands of consecutive nulls.