Make NullReport a public class

This PR is a response to @davidfowl 's previous feedback at
https://github.com/aspnet/KRuntime/commit/ae336d911251223b876d84fa7e85f9ae26a0e1f1#commitcomment-7264560

Making `NullReport` public is better than making it private because scaffolding wants to reuse `NullReport`.