Update scaffolders to support nullable reference types

[Nullable reference types was a feature introduced in C# 8](https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references). In .NET 6 we intend to enable it for new projects by default (see dotnet/templating#3359).

Any code emitted into a project by scaffolders should ensure that it doesn't introduce nullability warnings given the configuration of nullable reference types in the target project. This might mean updating the code to be "safe" for inclusion when nullable is on or off, or making the scaffolder emit different code depending on whether the project's nullable context configuration (i.e. the value of the `` property in the _.csproj_ file).

@vijayrkn