dropStack now returns more null stacks

The idea is that you can setInventorySlotContents(slotnumber,
dropStack(inventory.getStackInSlot(slotnumber)));. Damn, that looks
awkward.

But seriously though, being able to simply set the new inventory
contents to the value of dropStack(old contents) should be nifty.
Hopefully this doesn't break Item Routers or whatever.