[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. 
Is there a legitimate case where a Iterator[Array[Byte]] will contain a null?  I was hoping we'd only have to worry about nulls in Iterator[String].