[SPARK-2279] Added emptyRDD method to Java API

Added emptyRDD method to Java API with tests.

Author: bpaulin <bob@bobpaulin.com>

Closes #1597 from bobpaulin/SPARK-2279 and squashes the following commits:

5ad57c2 [bpaulin] [SPARK-2279] Added emptyRDD method to Java API