Fixes milk runtime in on_mob_life() (and maybe other reagents)

[Birdtalon]

About the pull request

Seen this runtime a couple of times now with milk specifically. But this situation I believe could also occur in other reagent/on_mob_life()

holder being null can occur if we qdel the reagent during remove_reagent() and we then try to call procs on the now null holder further down the thread of on_mob_life(). We now check again if !holder after calling remove_reagent() at
/datum/reagent level which will cause us to return early in on_mob_life() which should call parent when implemented and not operate on a null holder

[2023-12-28 17:57:27.386] runtime error: Cannot execute null.remove reagent().
 - proc name: on mob life (/datum/reagent/drink/milk/on_mob_life)
 -   source file: code/modules/reagents/chemistry_reagents/drink.dm,168
 -   usr: null
 -   src: Milk (/datum/reagent/drink/milk)
 -   call stack:
 - Milk (/datum/reagent/drink/milk): on mob life(Gilles Liebreich (/mob/living/carbon/human), 0, 2)
 - /datum/reagents (/datum/reagents): metabolize(Gilles Liebreich (/mob/living/carbon/human), 0, 2)
 - Gilles Liebreich (/mob/living/carbon/human): handle chemicals in body(2)
 - Gilles Liebreich (/mob/living/carbon/human): Life(2)
 - Human Life (/datum/controller/subsystem/human): fire(0)
 - Human Life (/datum/controller/subsystem/human): ignite(0)
 - Master (/datum/controller/master): RunQueue()
 - Master (/datum/controller/master): Loop(2)
 - Master (/datum/controller/master): StartProcessing(0)
 - 

Explain why it's good for the game

Testing Photographs and Procedure

Screenshots & Videos

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Changelog

🆑
fix: Runtime with milk and possibly other reagent's on_mob_life()
/:cl:

[Drulikar]

Code changes look fine.

I was also able to reproduce the runtime on master, but not reproduce it on this branch.