Fix Z.DynamicLinq.SystemTextJson when property is null

Author: StefH

src/System.Linq.Dynamic.Core.SystemTextJson/Config/NormalizationNonExistingPropertyBehavior.cs

@@ -1,7 +1,7 @@
 namespace System.Linq.Dynamic.Core.SystemTextJson.Config;
 
 /// <summary>
-/// Specifies the behavior to use when setting a property vlue that does not exist or is missing during normalization.
+/// Specifies the behavior to use when setting a property value that does not exist or is missing during normalization.
 /// </summary>
 public enum NormalizationNonExistingPropertyBehavior
 {

src/System.Linq.Dynamic.Core.SystemTextJson/Utils/NormalizeUtils.cs

@@ -104,10 +104,11 @@ private static JsonObject NormalizeObject(JsonObject source, Dictionary<string,
             {
                 if (source.ContainsKey(key))
                 {
+                    var value = source[key];
 #if NET8_0_OR_GREATER
-                    result[key] = source[key]!.DeepClone();
+                    result[key] = value?.DeepClone();
 #else
-                    result[key] = JsonNode.Parse(source[key]!.ToJsonString());
+                    result[key] = value != null ? JsonNode.Parse(value.ToJsonString()) : null;
 #endif
                 }
                 else

test/System.Linq.Dynamic.Core.NewtonsoftJson.Tests/NewtonsoftJsonTests.cs

@@ -1,5 +1,4 @@
-using System.Linq.Dynamic.Core.Exceptions;
-using System.Linq.Dynamic.Core.NewtonsoftJson.Config;
+using System.Linq.Dynamic.Core.NewtonsoftJson.Config;
 using FluentAssertions;
 using Newtonsoft.Json.Linq;
 using Xunit;
@@ -13,11 +12,13 @@ public class NewtonsoftJsonTests
         [
             {
                 "Name": "John",
-                "Age": 30
+                "Age": 30,
+                "IsNull": null
             },
             {
                 "Name": "Doe",
-                "Age": 40
+                "Age": 40,
+                "AlsoNull": null
             }
         ]
         """;

test/System.Linq.Dynamic.Core.SystemTextJson.Tests/SystemTextJsonTests.cs

@@ -17,11 +17,13 @@ public class SystemTextJsonTests
         [
             {
                 "Name": "John",
-                "Age": 30
+                "Age": 30,
+                "IsNull": null
             },
             {
                 "Name": "Doe",
-                "Age": 40
+                "Age": 40,
+                "AlsoNull": null
             }
         ]
         """;