Merge "Support empty histogram from the API" from Amnon

"This series support API calls that returns an empty histogram. This is a
 typical scenario with counters that are not implemented yet, for example range
 latency.  The trigger for this series is the nodetoold proxyhistograms command

 After this series:
 ./bin/nodetool proxyhistograms
 proxy histograms
 Percentile      Read Latency     Write Latency     Range Latency
                     (micros)          (micros)          (micros)
 50%                654949.00         315852.00               NaN
 75%               8409007.00        4055269.00               NaN
 95%              20924300.00       17436917.00               NaN
 98%              25109160.00       20924300.00               NaN
 99%              25109160.00       25109160.00               NaN
 Min                 11865.00          11865.00               NaN
 Max              25109160.00       25109160.00               NaN"

src/main/java/com/cloudius/urchin/api/APIClient.java

@@ -677,6 +677,9 @@ public class APIClient {
             MultivaluedMap<String, String> queryParams) {
         JsonObject obj = getJsonObj(string, queryParams);
         JsonArray arr = obj.getJsonArray("buckets");
+        if (arr == null) {
+            return new long[0];
+        }
         long res[] = new long[arr.size()];
         for (int i = 0; i< arr.size(); i++) {
             res[i] = arr.getJsonNumber(i).longValue();

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;

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];