Add nullable support

[kenkendk]

After the update to a modern C# version, the libraries should be updated one-by-one to support nullable notation.

To avoid long-standing PRs, there should be one or two libraries in each PR.
Simply add:

<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

To the .csproj file and fix the type annotations to correctly support null-able.

[bigboybamo]

Hi @kenkendk If this is still open, I'd like to pick this up

[kenkendk]

@bigboybamo Awesome!

I am not aware of any progress on the issue, so you can start where you like :)
I would recommend not trying to take on the whole codebase, but start with some of the core libraries (Duplicati.Library.Utility, Duplicati.Library.AutoUpdater, Duplicati.Library.License, Duplicati.Library.Localization) and get a PR merged with these first.

Starting "bottom-up" will make less back-trips to fix things once the libraries are nullable enabled.

[bigboybamo]

Hi @kenkendk I started with the Utility Library. I have the type annotations in a few files but I want to be sure I am on the right track (and its what you want) Can I open a small PR so you look at the changes? Then, maybe if its fine I can continue