avoid Nullable return type?

[runenilsenoe]

this is a bit confusing: https://github.com/DataObjects-NET/dataobjects-net/blob/2be511d0e5083e1b2cd58151e03d14a1d6604e5c/Orm/Xtensive.Orm/Orm/QueryEndpoint.cs#L349 The .Single inside will throw hence the nullability of the function is confusing