I like @RezaRahmati's suggestion, but an alternative is:
d.QtyOnOrder = db.DieOrders.Where(c=>c.DrawDie.SizeUS==d.SizeUS && d.QtyOpen.HasValue)
.Sum(c => c.QtyOpen);
If all of the QtyOpen
are null, then you are summing an empty list which will give you zero.
What I like about Reza's answer however, is that it makes it more explicit that you will set the result to zero if the sum is null.