CSHARP-5140: Coverity analysis defect 127372: Explicit null dereferenced.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/WindowMethodToAggregationExpressionTranslator.cs

@@ -18,6 +18,7 @@
 using System.Linq.Expressions;
 using System.Reflection;
 using MongoDB.Bson.Serialization;
+using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
 using MongoDB.Driver.Linq.Linq3Implementation.ExtensionMethods;
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
@@ -515,6 +516,8 @@ private static AstWindow ToAstWindow(SetWindowFieldsWindow window, object sortBy
 
         private static IBsonSerializer GetSortBySerializer(object sortBy, IBsonSerializer inputSerializer, BsonSerializerRegistry serializerRegistry)
         {
+            Ensure.IsNotNull(sortBy, nameof(sortBy));
+
             // use reflection to call GetSortBySerializerGeneric because we don't know TDocument until runtime
             var sortByType = sortBy.GetType();
             var documentType = sortByType.GetGenericArguments().Single();