Add nullable support about duplicati

[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

[kenkendk]

@bigboybamo Yes, of course, small things are easier to review :)

[bigboybamo]

@kenkendk alright I created a small PR #5207