APIClient: Support Empty histogram

An empty histogram can return a valid response from the API but without
any buckets.
This is a valid scenario and common for counters of features that are
not supported yet.
In those cases, the APIClient should return a zero length array.

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

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();