[Feature Request] Enable nullable type checks

[filzrev]

Currently BenchmarkDotNet using <Nullable>annotations</Nullable> setting.

Is there a plan to migrate to use <Nullable>enable</Nullable> in future?

Migration path
Currently 828 errors are raised when setting <Nullable>enable</Nullable> on solution-level.
Therefore, it might be better to migrate with following steps.

1. Enable nullable check with #nullable enable setting per-files. and fix build errors.
2. When entire code is refactored. Migrate to use <Nullable>enable</Nullable>. And remove #nullable enable settings

Background
When accessing ExecuteResult::StandardOutput property.
It's expected to be non-null value is returned.
But it returns null in some cases (When used constructor that don't initialize StandardOutput property)