fix: Ref: Sailthru support - provide the message_id as null if not provided

Author: Antaris

libs/SailthruSDK/Purchase/UpsertPurchaseRequest.cs

@@ -70,7 +70,7 @@ public override void Write(Utf8JsonWriter writer, UpsertPurchaseRequest value, J
 
 					writer.WriteStringProperty("email", value.Email, options);
 					writer.WriteBooleanProperty("incomplete", value.Incomplete, options);
-					writer.WriteStringProperty("message_id", value.MessageId ?? "", options);
+					writer.WriteStringProperty("message_id", value.MessageId ?? default, options);
 
 					writer.WritePropertyName("items");
 					writer.WriteStartArray();