feat: Add Nullable support

[maxkoshevoi]

Fixes

• Add nullable annotations (C# 8 nullable support)
• Initialize (some) collection properties
• Style fixes related to new C# capabilities (e.g. default(T) -> default)
• ~~Fix httpErrorAsException argument not used in SendGridClient.cs~~

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[maxkoshevoi]

~~Ok, I'm not sure how to build this thing using latest .net sdk (6.0). Could someone please update the PR to do so? https://github.com/sendgrid/sendgrid-csharp/pull/1110 might be enough though.~~

Update: Never mind, reverted C#9 changes