[SPARK-8373] [PYSPARK] Add emptyRDD to pyspark and fix the issue when calling sum on an empty RDD

This PR fixes the sum issue and also adds `emptyRDD` so that it's easy to create a test case.

Author: zsxwing <zsxwing@gmail.com>

Closes #6826 from zsxwing/python-emptyRDD and squashes the following commits:

b36993f [zsxwing] Update the return type to JavaRDD[T]
71df047 [zsxwing] Add emptyRDD to pyspark and fix the issue when calling sum on an empty RDD