InMemory: A better implementation of DefaultIfEmpty

[smitpatel]

Currently DefaultIfEmpty marks everything coming out as nullable and inject a null values valueBuffer.
But certain correlated query it fails. In following case, City comes from outer and even though there is DefaultIfEmpty, we still read value from outer valueBuffer.

            return AssertQuery<Customer, Order>(
                isAsync,
                (cs, os) => from c in cs
                            from o in os.Where(o => c.CustomerID == o.CustomerID).Select(o => c.City).DefaultIfEmpty()
                            select new
                            {
                                c,
                                o
                            },
                entryCount: 91);

[ajcvickers]

We recommend against using the in-memory provider for testing--see Testing EF Core Applications. While we have no plans to remove the in-memory provider, we will not be adding any new features to this provider because we believe valuable development time is better spent in other areas. When feasible, we plan to still fix regressions in existing behavior.