Allow repartitioning empty RDDs to zero partitions.

Author: darabos

core/src/main/scala/org/apache/spark/rdd/CoalescedRDD.scala

@@ -78,7 +78,8 @@ private[spark] class CoalescedRDD[T: ClassTag](
     balanceSlack: Double = 0.10)
   extends RDD[T](prev.context, Nil) {  // Nil since we implement getDependencies
 
-  require(maxPartitions > 0, s"Number of partitions ($maxPartitions) must be positive.")
+  require(maxPartitions > 0 || maxPartitions == prev.partitions.length,
+    s"Number of partitions ($maxPartitions) must be positive.")
 
   override def getPartitions: Array[Partition] = {
     val pc = new PartitionCoalescer(maxPartitions, prev, balanceSlack)