Properly handle `@NullUnmarked` generic types

[msridhar]

Right now, we report an error for code like java.util.function.Function<@Nullable String,String> f = ... since we think Function has a non-null upper bound. But, since we currently treat Function as @NullUnmarked we shouldn't be doing this check in the first place. We need to tweak some of our JSpecify checking to detect when relevant code is @NullUnmarked` and bail out.

[msridhar]

FYI @akulk022

[msridhar]

I think this was fixed by #958