Minor Description
Gremlin.NET should enable nullable checks to clearly communicate to users what can be null in Gremlin.NET and also to possibly find possible NullReferenceExceptions easier in our own code.
The recommendation from Microsoft is to update libraries during the so-called "nullable rollout phase" which ends in November with the expected release of .NET 5 which should have nullable checks enabled by default.
We can add .NET Core 3.1 as an additional target framework to get the nullable checks and still also target .NET Standard to stay platform independent and to also still support .NET Framework and older .NET Core versions.