fixes runtime when trying to remove a nonexistent source ID via RemoveInvisibility()

[ShizCalev]

[2024-05-15 18:24:21.291] RUNTIME: runtime error: cannot read from list
 - proc name: RemoveInvisibility (/atom/proc/RemoveInvisibility)
 -   source file: code/game/atom/atom_invisibility.dm,66
 -   usr: null
 -   src: the turret (/obj/machinery/porta_turret/ai)
 -   src.loc: the floor (187,33,3) (/turf/open/floor/circuit)
 -   call stack:
 - the turret (/obj/machinery/porta_turret/ai): RemoveInvisibility(/obj/machinery/porta_turret_co... (/obj/machinery/porta_turret_cover))
 - the turret (/obj/machinery/porta_turret_cover): Destroy(0)
 - qdel(the turret (/obj/machinery/porta_turret_cover), 0)
 - the turret (/obj/machinery/porta_turret/ai): atom break("melee")
 - the turret (/obj/machinery/porta_turret/ai): take damage(20, "brute", "melee", 1, 4, 0)
 - the turret (/obj/machinery/porta_turret/ai): take damage(40, "brute", "melee", 1, 4, 0)
 - the turret (/obj/machinery/porta_turret/ai): attack generic(the turret (/mob/living/simple_animal/hostile/mimic/copy/machine), 40, "brute", "melee", 1, 0)
 - the turret (/obj/machinery/porta_turret/ai): attack animal(the turret (/mob/living/simple_animal/hostile/mimic/copy/machine), null)
 - the turret (/mob/living/simple_animal/hostile/mimic/copy/machine): AttackingTarget(the turret (/obj/machinery/porta_turret/ai))
 - the turret (/mob/living/simple_animal/hostile/mimic/copy/machine): AttackingTarget(the turret (/obj/machinery/porta_turret/ai))
 - the turret (/mob/living/simple_animal/hostile/mimic/copy/machine): MeleeAction(1)
 - the turret (/mob/living/simple_animal/hostile/mimic/copy/machine): MoveToTarget(/list (/list))
 - the turret (/mob/living/simple_animal/hostile/mimic/copy/machine): handle automated action()
 - NPC Pool (/datum/controller/subsystem/npcpool): fire(0)
 - NPC Pool (/datum/controller/subsystem/npcpool): ignite(0)
 - Master (/datum/controller/master): RunQueue()
 - Master (/datum/controller/master): Loop(2)
 - Master (/datum/controller/master): StartProcessing(0)
 -