Add missing #nullable in test file

Author: benjaminpetit

src/Mvc/test/WebSites/BasicWebSite/Controllers/CustomServiceApiController.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#nullable enable
+
 using Microsoft.AspNetCore.Mvc;
 
 namespace BasicWebSite;