[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. 
Right, but that's a private API, it doesn't matter. Does our own code do it?

Basically I'm worried that this significantly complicates our code for something that shouldn't happen. I'd rather have an NPE if our own code later passes nulls here (cause it really shouldn't be doing that since we control everything we pass in).