Add nullable to DataProtection

[pranavkm]

No description provided.

[ghost]

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[BrennanConroy]

Why is there a seemingly random choice of enable or annotations for each csproj, and why is this the only multi-targeted one that does this logic?

[pranavkm]

In general, we can only turn on nullable = annotations for ns2.0 projects. All ns2.0 APIs are oblivious, so if you rely on BCL's api for nullability, it doesn't work. In this case, we rely on annotations in Debug.Assert. Most of the other projects here are implementation heavy, changing it is going to take a lot of work, which is they are annotation only.