Handle empty aggregation (#3722)

Closes #3717

Author: russcam

src/Nest/Aggregations/AggregateFormatter.cs

@@ -87,8 +87,11 @@ public IAggregate Deserialize(ref JsonReader reader, IJsonFormatterResolver form
 		private IAggregate ReadAggregate(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
 		{
 			reader.ReadIsBeginObjectWithVerify();
-
 			IAggregate aggregate = null;
+
+			if (reader.ReadIsEndObject())
+				return aggregate;
+
 			var propertyName = reader.ReadPropertyNameSegmentRaw();
 			Dictionary<string, object> meta = null;
 

src/Tests/Tests.Reproduce/GithubIssue3717.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Text;
+using Elastic.Xunit.XunitPlumbing;
+using Elasticsearch.Net;
+using FluentAssertions;
+using Nest;
+
+namespace Tests.Reproduce
+{
+	public class GithubIssue3717
+	{
+		[U]
+		public void CanDeserializeEmptyAggregation()
+		{
+			var json = @"{
+  ""took"" : 2,
+  ""timed_out"" : false,
+  ""_shards"" : {
+    ""total"" : 3,
+    ""successful"" : 3,
+    ""skipped"" : 0,
+    ""failed"" : 0
+  },
+  ""hits"" : {
+    ""total"" : {
+      ""value"" : 10000,
+      ""relation"" : ""gte""
+    },
+    ""max_score"" : null,
+    ""hits"" : [ ]
+  },
+  ""aggregations"" : {
+    ""geo_bounds#viewport"" : { }
+  }
+}";
+
+			var bytes = Encoding.UTF8.GetBytes(json);
+
+			var pool = new SingleNodeConnectionPool(new Uri("http://localhost:9200"));
+			var connectionSettings = new ConnectionSettings(pool, new InMemoryConnection(bytes));
+			var client = new ElasticClient(connectionSettings);
+
+			Func<SearchResponse<TestEntity>> response = () => client.Search<TestEntity>(s => s
+				.Size(0)
+				.Index("issue")
+				.Aggregations(q => q
+					.GeoBounds("viewport", t => t
+						.Field(f => f.TestProperty)))
+			);
+
+			response.Should().NotThrow();
+		}
+
+		public class TestEntity {
+			public string TestProperty { get; set; }
+		}
+	}
+}