Nullable reference types

__Proposal:__
.NET libraries are moving towards using [Nullable reference types](https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references). Are you planning to add them to InfluxDB client.

__Current behavior:__
No nullable annotations in the libraries.

__Desired behavior:__
Nullable annotations everywhere in the InfluxDB client.

__Use case:__
Usage of nullable reference types in the library enables developers to write more stable (null-safe) code.

No source files, tests, or entry points are named. Start by surveying the client’s C# library projects and existing build and test setup, then determine the scope of nullable annotations across the libraries. Done means the agreed libraries consistently expose nullable annotations while the existing test suite and build pass.

Written by the indexing model from the issue text.