fix on empty buckets (#3068)

Change `do { ... } while` loop to `while` loop so the deserialisation doesn't crash when receiving an empty aggregation bucket.

Author: russcam

src/Nest/Aggregations/AggregateJsonConverter.cs

@@ -498,14 +498,14 @@ private IAggregate GetMultiBucketAggregate(JsonReader reader, JsonSerializer ser
 			{
 				reader.Read();
 				var aggs = new Dictionary<string, IAggregate>();
-				do
+				while (reader.TokenType != JsonToken.EndObject)
 				{
 					var name = reader.Value.ToString();
 					reader.Read();
 					var innerAgg = this.ReadAggregate(reader, serializer);
 					aggs.Add(name, innerAgg);
 					reader.Read();
-				} while (reader.TokenType != JsonToken.EndObject);
+				}
 
 				reader.Read();
 				return new FiltersAggregate(aggs);