[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. 
It's easy to call private API in Python, log4j can easily to do this via reflection, just like we do in PySpark.

For example, 
{{{
java_func = sc._jvm.com.xxx.XClass.XFunc
scala_rdd = py_rdd._jrdd.rdd().map(java_func)
java_rdd = sc._jvm.PythonRDD(scala_rdd, '', env, includes, ...).asJavaRDD()
rdd = RDD(java_rdd, sc, UTF8Deserializer())
}}}

If java_func can return null, then it will have NPE.