Write null when ISort or SortKey is null (#4132)

This commit writes null when ISort or ISort.SortKey is null. Simply returning without writing a value can result in incorrectly formatted JSON.

Closes #4128

(cherry picked from commit 71c8aae)

Author: russcam

src/Nest/CommonOptions/Sorting/SortFormatter.cs

@@ -75,7 +75,10 @@ public ISort Deserialize(ref JsonReader reader, IJsonFormatterResolver formatter
 		public void Serialize(ref JsonWriter writer, ISort value, IJsonFormatterResolver formatterResolver)
 		{
 			if (value?.SortKey == null)
+			{
+				writer.WriteNull();
 				return;
+			}
 
 			writer.WriteBeginObject();
 			var settings = formatterResolver.GetConnectionSettings();