Enabling null checks in compile time

[Eneuman]

@domaindrivendev Would you be intrested in accepting a PR for enabling null checks in compile time? I could enabling <-Nullable->enable</-Nullable-> for all the projects and fix any nullabillity issues discovered?

I ran into a issue where I missed to check for null in the Nullable Reference type PR, and theese kinds if misses can be avoided by enabling nullable checks at compile time :)

[liesahead]

Any updates?

[martincostello]

We'd be open to a PR to add this, but if it's a big change it might have to wait for a release or two to be merged as we'd likely want to get bug fixes out without causing breaking changes at the same time.

[martincostello]

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/45a2cb71996868e71dc5cad9aef6f5e7cac533ba/Directory.Build.props#L12-L15