Food can have null reagents

Created by: gbasood

Cannot read null.total_volume at snacks.dm:152
 proc name: attack (/obj/item/weapon/reagent_containers/food/snacks/attack)
  source file: snacks.dm,152
  usr: Tihati (/mob/living/carbon/human)
  src: the donut (/obj/item/weapon/reagent_containers/food/snacks/donut/normal)
  call stack:
the donut (/obj/item/weapon/reagent_containers/food/snacks/donut/normal): attack(Tihati (/mob/living/carbon/human), Tihati (/mob/living/carbon/human), null, 0)

Also seen on cake slices