[SPARK-4304] [PySpark] Fix sort on empty RDD

This PR fix sortBy()/sortByKey() on empty RDD.

This should be back ported into 1.1/1.2

Author: Davies Liu <davies@databricks.com>

Closes #3162 from davies/fix_sort and squashes the following commits:

84f64b7 [Davies Liu] add tests
52995b5 [Davies Liu] fix sortByKey() on empty RDD