fix: null default value

Author: VisualBean

src/LEGO.AsyncAPI/Models/Avro/AvroField.cs

@@ -67,7 +67,18 @@ public void SerializeV2(IAsyncApiWriter writer)
             writer.WriteOptionalProperty("name", this.Name);
             writer.WriteOptionalObject("type", this.Type, (w, s) => s.SerializeV2(w));
             writer.WriteOptionalProperty("doc", this.Doc);
-            writer.WriteOptionalObject("default", this.Default, (w, s) => w.WriteAny(s));
+            writer.WriteOptionalObject("default", this.Default, (w, s) =>
+            {
+                if (s.TryGetValue(out string value) && value == "null")
+                {
+                    w.WriteNull();
+                }
+                else
+                {
+                    w.WriteAny(s);
+                }
+            });
+
             if (this.Order != AvroFieldOrder.None)
             {
                 writer.WriteOptionalProperty("order", this.Order.GetDisplayName());

test/LEGO.AsyncAPI.Tests/Models/AvroSchema_Should.cs

@@ -10,6 +10,29 @@ namespace LEGO.AsyncAPI.Tests.Models
 
     public class AvroSchema_Should
     {
+        [Test]
+        public void Serialize_WithDefaultNull_SetJsonNull()
+        {
+            var input = """
+            type: record
+            name: User
+            namespace: Producer
+            doc: ESP Schema validation test
+            fields:
+              - name: userId
+                type: int
+              - name: userEmail
+                type:
+                  - null
+                  - string
+                default: null
+            """;
+
+            var model = new AsyncApiStringReader().ReadFragment<AvroSchema>(input, AsyncApiVersion.AsyncApi2_0, out var diag);
+            var reserialized = model.SerializeAsJson(AsyncApiVersion.AsyncApi2_0);
+            reserialized.Should().Contain("default\": null");
+        }
+
         [Test]
         public void Deserialize_WithMetadata_CreatesMetadata()
         {