[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. 
Array[Byte] is similar to String, null can be generated by user's functions or RDDs, just like

RDD[String].map(x => if (x != null) x.toArray else x)