MaterialData#toItemStack without amount should make 1 item by default

An Item entity with 0 amount in its ItemStack causes a trouble in Minecraft world; players can pick up the item but cannot do anything with it. Current implementation of MaterialData#toItemStack with no amount argument doesn't make sense, so this commit changes the default value to let it have a meaning.

Another solution: just remove toItemStack without amount but keep toItemStack with amount.