EstimatedHistogram: Support empty histogram

When creating an estimated histogram from buckets it is a valid option
to get a zero size array as the buckets array.

In that case the newOffsets method would get a negative value for its
size, which should result in a zero length array of offsets.

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

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

@@ -74,6 +74,9 @@ public class EstimatedHistogram {
     }
 
     private static long[] newOffsets(int size) {
+        if (size <= 0) {
+            return new long[0];
+        }
         long[] result = new long[size];
         long last = 1;
         result[0] = last;