Add MaybeNull attribute to EntityFrameworkQueryableExtensions.cs

[dannyBies]

Fixes #21793

[roji]

The [MaybeNull] attributes you've added to SingleOrDefaultAsync and the rest applies to the entire return value of those methods, i.e. to the Task<TSource>, rather than to the TSource which is what you're looking for. I'm not currently aware of a way to properly annotate these methods for nullability, but the C# team is aware of this (/cc @cartermp).

I hope we'll have the time to annotate all of EF Core's API surface in the 6.0 timeframe - that would include this as well.

[dannyBies]

Good point, I hadn't thought of that. It would indeed be nice to have it annotated as it can result in subtle bugs right now.

[roji]

Agreed, hopefully there will be a solution for annotating this soon.