Correctly serialize _geo_distance sort

This commit updates the SortFormatter to emit a value separator when serializing a _geo_distance only when
the IGeoDistanceSort contains properties with values. This is determined by checking the serialized bytes
are more than 2 i.e. an empty JSON object, {}.

Add additional _geo_distance sort to SortUsage tests to cover when IGeoDistanceSort contains/doesn't contain properties with
values.

(cherry picked from commit 0564747)

Author: russcam

src/Nest/CommonOptions/Sorting/SortFormatter.cs

@@ -103,7 +103,8 @@ public void Serialize(ref JsonWriter writer, ISort value, IJsonFormatterResolver
 					for (var i = buffer.Offset; i < buffer.Count - 1; i++)
 						writer.WriteRawUnsafe(buffer.Array[i]);
 
-					if (buffer.Count > 0)
+					// does the IGeoDistanceSort have other properties set i.e. is it more than simply {} ?
+					if (buffer.Count > 2)
 						writer.WriteValueSeparator();
 
 					writer.WritePropertyName(settings.Inferrer.Field(geo.Field));

src/Tests/Tests/Search/Request/SortUsageTests.cs

@@ -79,6 +79,25 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 						}
 					},
 					new
+					{
+						_geo_distance = new
+						{
+							locationPoint = new[]
+							{
+								new
+								{
+									lat = 70.0,
+									lon = -70.0
+								},
+								new
+								{
+									lat = -12.0,
+									lon = 12.0
+								}
+							}
+						}
+					},
+					new
 					{
 						_script = new
 						{
@@ -127,6 +146,10 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 					.Mode(SortMode.Min)
 					.Points(new GeoLocation(70, -70), new GeoLocation(-12, 12))
 				)
+				.GeoDistance(g => g
+					.Field(p => p.LocationPoint)
+					.Points(new GeoLocation(70, -70), new GeoLocation(-12, 12))
+				)
 				.Script(sc => sc
 					.Type("number")
 					.Ascending()
@@ -175,6 +198,11 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 						Mode = SortMode.Min,
 						Points = new[] { new GeoLocation(70, -70), new GeoLocation(-12, 12) }
 					},
+					new GeoDistanceSort
+					{
+						Field = "locationPoint",
+						Points = new[] { new GeoLocation(70, -70), new GeoLocation(-12, 12) }
+					},
 					new ScriptSort
 					{
 						Type = "number",