enable `PropertyGroup.Nullable`

[BryanWilhite]

see:

• https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/nullable-reference-types
• https://devblogs.microsoft.com/dotnet/try-out-nullable-reference-types/
• https://docs.microsoft.com/dotnet/csharp/nullable-references

[BryanWilhite]

yet another challenge: netstandard2.0 does not support nullable reference types (because it is C# 7.0)

blindly, i have upgraded the Core to netstandard2.1 (C# 8.0) but i do not know which version of .NET Framework supports it

https://en.wikipedia.org/wiki/List_of_.NET_libraries_and_frameworks#cite_ref-9

this info is either out of date or i need to drop support for .NET Framework via .NET Standard