Fixes supermatter sliver objective being given if there is no supermatter engine (#30032)

* Adds /engine variants of supermatter_shard

* Makes maps use /engine supermatter_shard variants

* Adds a framework for ensuring an objective_item exists before giving it as a target

* Fixes supermatter sliver objective being given with no supermatter engine on the map

* Works around BYOND stupidity

code/modules/power/supermatter/supermatter.dm

@@ -55,6 +55,8 @@
 #define SUPERMATTER_DANGER_PERCENT 50
 #define SUPERMATTER_WARNING_PERCENT 100
 
+GLOBAL_DATUM(main_supermatter_engine, /obj/machinery/power/supermatter_shard)
+
 /obj/machinery/power/supermatter_shard
 	name = "supermatter shard"
 	desc = "A strangely translucent and iridescent crystal that looks like it used to be part of a larger structure."
@@ -131,6 +133,7 @@
 	var/produces_gas = TRUE
 	var/obj/effect/countdown/supermatter/countdown
 
+	var/is_main_engine = FALSE
 
 /obj/machinery/power/supermatter_shard/Initialize()
 	. = ..()
@@ -143,7 +146,8 @@
 	radio.listening = 0
 	radio.recalculateChannels()
 	investigate_log("has been created.", INVESTIGATE_SUPERMATTER)
-
+	if(is_main_engine)
+		GLOB.main_supermatter_engine = src
 
 /obj/machinery/power/supermatter_shard/Destroy()
 	investigate_log("has been destroyed.", INVESTIGATE_SUPERMATTER)
@@ -151,6 +155,8 @@
 	QDEL_NULL(radio)
 	GLOB.poi_list -= src
 	QDEL_NULL(countdown)
+	if(is_main_engine && GLOB.main_supermatter_engine == src)
+		GLOB.main_supermatter_engine = null
 	. = ..()
 
 /obj/machinery/power/supermatter_shard/examine(mob/user)
@@ -584,6 +590,9 @@
 		else
 			L.show_message("<span class='italics'>You hear an unearthly ringing and notice your skin is covered in fresh radiation burns.</span>", 2)
 
+/obj/machinery/power/supermatter_shard/engine
+	is_main_engine = TRUE
+
 // When you wanna make a supermatter shard for the dramatic effect, but
 // don't want it exploding suddenly
 /obj/machinery/power/supermatter_shard/hugbox
@@ -599,6 +608,9 @@
 	gasefficency = 0.15
 	explosion_power = 35
 
+/obj/machinery/power/supermatter_shard/crystal/engine
+	is_main_engine = TRUE
+
 /obj/machinery/power/supermatter_shard/proc/supermatter_pull(turf/center, pull_range = 10)
 	playsound(src.loc, 'sound/weapons/marauder.ogg', 100, 1, extrarange = 7)
 	for(var/atom/P in orange(pull_range,center))