Fix Utf8JsonWriterHelper to not ignore null dictionary values

This would avoid sending any null values from StrawberryShake in such a way that if you wanted to set a null value on a request, you could not.  They will always come back as unassigned.

src/HotChocolate/AspNetCore/src/Transport.Abstractions/Serialization/Utf8JsonWriterHelper.cs

@@ -194,13 +194,15 @@ private static void WriteDictionary(
 
         foreach (var item in dict)
         {
+            writer.WritePropertyName(item.Key);
             if (item.Value is null)
             {
-                continue;
+                writer.WriteNullValue();
+            }
+            else
+            {
+                WriteFieldValue(writer, item.Value);
             }
-
-            writer.WritePropertyName(item.Key);
-            WriteFieldValue(writer, item.Value);
         }
 
         writer.WriteEndObject();

src/HotChocolate/AspNetCore/test/Transport.Http.Tests/OperationRequestTests.cs

@@ -0,0 +1,37 @@
+using System.Text;
+using System.Text.Json;
+using CookieCrumble;
+using Moq;
+
+namespace HotChocolate.Transport.Http.Tests;
+
+public class OperationRequestTests
+{
+    [Fact]
+    public async Task Should_WriteNullValues()
+    {
+        // arrange
+        var request = new OperationRequest(
+            null,
+            "abc",
+            "myOperation",
+            variables: new Dictionary<string, object?>()
+            {
+                ["abc"] = "def",
+                ["hij"] = null
+            });
+
+        using var memory = new MemoryStream();
+        await using var writer = new Utf8JsonWriter(memory);
+
+        // act
+        request.WriteTo(writer);
+        await writer.FlushAsync();
+
+        // assert
+        var result = Encoding.UTF8.GetString(memory.ToArray());
+        Assert.Equal(
+            """{"id":"abc","operationName":"myOperation","variables":{"abc":"def","hij":null}}""",
+            result);
+    }
+}