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#

[Numpsy]

A small Q. with regards to #139 and the netstandard2.1 project change:

Could the dependencies be reduced in the 2.1 target, even if the existing 2.0 target is left the same?

[Numpsy]

/meant to put that comment in the PR :-(

[rafaelsc]

I had a quick look and look like none of this PackageReference are required for any netstandard (even the netstandard1.3).
I will test more asap and probably create a new PR if we can clean up this PackageReference

[rafaelsc]

Just created a new PR #172 to remove unnecessary references in the project.