Fix excess orders stopping other orders' creation

[lL1l1]

Solves the "strange bug" with tactical missile / billy nuke and teleport: FAForever#5679
First it wasn't creating the orders because 7 base orders + 3 engineering orders + 1 OC + 2 missile + 1 tele = 14, which is more than the 12 allowed to be created.
After fixing that, there is a bug where orders are created out of all available orders using limited order-assigned slots. This meant that it could grab a nil slot for an available order, since that order didn't get assigned in the slot checking beforehand. This is fixed with a nil check.
Ideally there would be a priority system for what orders are given slots, but that isn't worth the effort given how rarely you select units with enough orders to do that (needs cross-faction, cross-theater, and/or very expensive units).