Add nullable annotations to NuGet.Frameworks

[zivkan]

NuGet Product(s) Affected

NuGet SDK

Current Behavior

Currently NuGet.Frameworks doesn't have annotations to allow the compiler to work with the compiler's "nullable reference types" feature.

Desired Behavior

Add null annotations, so the compiler knows when null might be returned, and when it's definitely not.

Additional Context

Anyone using NuGet.Frameworks in their project might find that upgrading to the newer version of NuGet.Frameworks might result in compiler warnings or errors, depending on if your project opts into nullable checks.

Additionally, it may be that while annotating the project that null-related bugs are found and fixed. Any app that depends on the behaviour considered buggy might find their app behaves differently since NuGet.Frameworks might throw exceptions when it previously didn't, or have a different result for the same inputs.