Update UnionFormatter to attempt TSecond if TFirst returns a null

This is to account for a Union<GeoCoordinate, DateMath> in the distance_query feature, which can result in an unwanted null in the GeoCoordinateFormatter

Author: codebrain

src/Nest/CommonAbstractions/Union/UnionFormatter.cs

@@ -5,16 +5,37 @@ namespace Nest
 {
 	internal class UnionFormatter<TFirst, TSecond> : IJsonFormatter<Union<TFirst, TSecond>>
 	{
+		private readonly bool _attemptTSecondIfTFirstIsNull;
+
+		public UnionFormatter()
+		{
+			_attemptTSecondIfTFirstIsNull = false;
+		}
+
+		public UnionFormatter(bool attemptTSecondIfTFirstIsNull)
+		{
+			_attemptTSecondIfTFirstIsNull = attemptTSecondIfTFirstIsNull;
+		}
+
 		public Union<TFirst, TSecond> Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
 		{
-			Union<TFirst, TSecond> union = null;
 			var segment = reader.ReadNextBlockSegment();
 			if (TryRead(ref segment, formatterResolver, out TFirst first))
-				union = first;
+			{
+				if (first == null && _attemptTSecondIfTFirstIsNull)
+				{
+					if (TryRead(ref segment, formatterResolver, out TSecond second))
+						return second;
+				}
+				else
+				{
+					return first;
+				}
+			}
 			else if (TryRead(ref segment, formatterResolver, out TSecond second))
-				union = second;
+				return second;
 
-			return union;
+			return null;
 		}
 
 		public void Serialize(ref JsonWriter writer, Union<TFirst, TSecond> value, IJsonFormatterResolver formatterResolver)

src/Nest/QueryDsl/Specialized/DistanceFeature/DistanceFeatureQuery.cs

@@ -77,7 +77,7 @@ public DistanceFeatureQueryDescriptor<T> Pivot(Distance pivot) =>
 
 	internal class DistanceFeatureQueryFormatter : IJsonFormatter<IDistanceFeatureQuery>
 	{
-		private static readonly UnionFormatter<GeoCoordinate, DateMath> OriginUnionFormatter = new UnionFormatter<GeoCoordinate, DateMath> ();
+		private static readonly UnionFormatter<GeoCoordinate, DateMath> OriginUnionFormatter = new UnionFormatter<GeoCoordinate, DateMath>(true);
 		private static readonly UnionFormatter<Distance, Time> PivotUnionFormatter = new UnionFormatter<Distance, Time>();
 
 		public void Serialize(ref JsonWriter writer, IDistanceFeatureQuery value, IJsonFormatterResolver formatterResolver)
@@ -101,7 +101,6 @@ public void Serialize(ref JsonWriter writer, IDistanceFeatureQuery value, IJsonF
 
 			writer.WritePropertyName("pivot");
 			PivotUnionFormatter.Serialize(ref writer, value.Pivot, formatterResolver);
-
 			writer.WriteValueSeparator();
 
 			if (value.Boost.HasValue)

src/Tests/Tests/QueryDsl/Specialized/DistanceFeature/DistanceFeatureTimeQueryUsageTests.cs

@@ -73,7 +73,7 @@ public DistanceFeatureDistanceQueryUsageTests(ReadOnlyCluster i, EndpointUsage u
 		};
 
 		protected override object QueryJson =>
-			new { distance_feature = new { boost = 1.1, field = "startedOn", origin = new { lat = 70.0, lon = -70.0 }, pivot = "100mi" } };
+			new { distance_feature = new { boost = 1.1, field = "leadDeveloper.location", origin = new [] { -70.0, 70.0 }, pivot = "100mi" } };
 
 		protected override QueryContainer QueryFluent(QueryContainerDescriptor<Project> q) => q
 			.DistanceFeature(rf => rf