Add support C# 8 nullable reference types

[rafaelsc]

Does this issue relate to a new feature or an existing bug?

• Bug
• New Feature

What is the target framework and operating system affected by this issue? Please see target frameworks & net standard matrix.

• netstandard2.1. NEW
• netCore 2.0
• netCore 1.0
• 4.7
• 4.6.x
• 4.5.x

Describe the solution you'd like
Enable nullable-references in Serilog and add update the codebase to support the nullable reference types in .NET Core 3.0 and 3.1.

Additional context
Links about the *Nullable reference types *:

• Nullable reference types (C# reference)
• Nullable reference types
• Introducing Nullable Reference Types in C#

[rafaelsc]

Related - serilog/serilog#1455

[jwdonahue]

I would really love to see this feature enabled soon. This looks scary:

  var logger = loggerMinimumLevelConfiguration
           !.Enrich?.With(enricher)
           ?.WriteTo.File(logPathFileName,
                   rollingInterval: interval,
                   outputTemplate: outputTemplate)
           ?.CreateLogger();
  Verify.IsNotNull(logger);

Is there anything I can do to help it along?

[Numpsy]

I think the nullability annotations should be enabled in the latest version now.