chore: null check for Serialize<T> (#165)

Author: VisualBean

src/LEGO.AsyncAPI/Models/AsyncApiSerializableExtensions.cs

@@ -109,6 +109,11 @@ public static void Serialize<T>(
                 throw new ArgumentNullException(nameof(stream));
             }
 
+            if (settings is null)
+            {
+                throw new ArgumentNullException(nameof(settings));
+            }
+
             var streamWriter = new FormattingStreamWriter(stream, settings.CultureInfo);
 
             IAsyncApiWriter writer = format switch