ratelimit: don't add json object if map is empty

An object map was added for each hash entry, even though there were no
pipes in that hash, resulting in a huge list of empty hashes [{},{} ...]
This fix does not add an object in the resulted array if the map is
empty.

(cherry picked from commit a37642a)

modules/ratelimit/ratelimit_helper.c

@@ -611,6 +611,8 @@ int rl_stats(mi_item_t *resp_obj, str * value)
 		if (!pipe_arr)
 			return -1;
 		for (i = 0; i < rl_htable.size; i++) {
+			if (map_size(rl_htable.maps[i]) == 0)
+				continue;
 			pipe_item = add_mi_object(pipe_arr, NULL, 0);
 			if (!pipe_item)
 				return -1;