Fallback ToListAsync() to .ToList() when IAsyncEnumerable is not available

[SergeiPavlov]

There are many non-Test cases where IAsyncEnumerable is not implemented
For example: nested .Select() in DO LINQ expressions:

var poData = await Session.Query.All<PurchaseOrder>()
            .Where(po => po.Id == purchaseOrderId)
            .Select(
                po => new {
                    InvoiceId = po.Invoice != null ? (long?)po.Invoice.Id : null,
                    Items = po.Items.Where(i => i.Active).Select(i => i.Id)
                })
            .SingleAsync();

They will fail without this change