[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. 
Someone opened the original PR and I had the same question there, it seemed unnecessary. The PR wasn't open to fix a bug as far as I know, or at least the person didn't report a particular case where it crashes.