[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. 
I was wrong; this _is_ tail-recursive.  If we only expect nulls to occur in iterators of strings, then I think we should be able to remove the null checking here.