Async method with nullable return type can return null

[jcouv]

Currently, this produces a warning, but it should not.

using System.Threading.Tasks;

public class C {
    public async Task<string?> M() {
        await Task.Delay(10);
        return null; // warning CS8625: Cannot convert null literal to non-nullable reference or unconstrained type parameter.
    }
}

Found in nullable dogfood. Assigned to @cston

[cston]

Duplicate of #23275.