Minecraftforge: Update IItemHandler.getStackInSlot documentation to forbid null ItemStacks

[rwtema]

I wrote the IItemHandler code for 1.10.2 when null stacks were still a thing. Now we have ItemStack.EMPTY and the getStackInSlot is marked as @Nonnull, this should be updated to say the returned value should never be null.

[liach]

Are the annotations fixed? Only the annotations really matter

[thiakil]

The javadoc shouldn't disagree with the annotation though.

I'd like to request that extractItem (and possibly insertItem) explicitly mention whether the returned stack can (or cannot) be modified. I've seen a few mods who return their internal stack on simulate, and dupes pop up when other mods modify that (because they expected it to be a copy already)

[HenryLoenwind]

Also, there's this bit of javadoc:

     * If the result is not null but the stack size is zero, then it represents
     * an empty slot that will only accept* a specific itemstack.

Stack size zero? Completely impossible.

[mezz]

Fixed here: bbd4d8d