Add nullable annotation to ResultContext<TOptions>.Result

[joegoldman2]

Fixes #45594.

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[halter73]

Thanks! This isn't even one of the white lies where something really is nullable right after construction, but it's not null later when application developers typically see it. It's most often null for the full lifetime of the context unless an event short circuits authentication.

I guess the reason no one noticed until now is that it's typically only read by framework code where we always null check it despite the previously incorrect annotation. It's definitely worth getting right though.