Added tests for joins, cogroups, and unions for EmptyRDD.

core/src/test/scala/spark/RDDSuite.scala

@@ -157,6 +157,14 @@ class RDDSuite extends FunSuite with LocalSparkContext {
       empty.reduce(_+_)
     }
     assert(thrown.getMessage.contains("empty"))
+
+    val emptyKv = new EmptyRDD[(Int, Int)](sc)
+    val rdd = sc.parallelize(1 to 2, 2).map(x => (x, x))
+    assert(rdd.join(emptyKv).collect().size === 0)
+    assert(rdd.rightOuterJoin(emptyKv).collect().size === 0)
+    assert(rdd.leftOuterJoin(emptyKv).collect().size === 2)
+    assert(rdd.cogroup(emptyKv).collect().size === 2)
+    assert(rdd.union(emptyKv).collect().size === 2)
   }
 
   test("cogrouped RDDs") {