Make nullable warnings a per file opt-in (#1842)

* Make nullable warnings a per file opt-in

* Remove unneeded compiler directives

* Remove compiler directive for User Data

test/Microsoft.SqlTools.ServiceLayer.UnitTests/QueryExecution/DbCellValueTests.cs

@@ -3,6 +3,8 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 //
 
+#nullable disable
+
 using System;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
 using NUnit.Framework;