[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. 
BTW apart from the stability issue above with catching our own bugs, the reason I'm commenting is that this change also adds some moderately tricky code in a fairly important code path, increasing the chance of adding new bugs. That doesn't seem worth it to me.