RecentEstimatedHistogram: Support empty histogram

The RecentEstimatedHistogram updates its value from the API with an
array of recent values.

This array can be empty, in that case the getBuckets method should just
return a zero size array.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

src/main/java/com/cloudius/urchin/utils/RecentEstimatedHistogram.java

@@ -50,6 +50,9 @@ public class RecentEstimatedHistogram extends EstimatedHistogram {
      * @return a long[] containing the current histogram difference buckets
      */
     public long[] getBuckets(long[] bucketData) {
+        if (bucketData.length == 0) {
+            return new long[0];
+        }
         final int len = buckets.length();
         long[] rv = new long[len];