Converts A && A.B into A?.B (#54342)

Implements the ?. operator, replacing code like A && A.B with A?.B

BYOND Ref:
When reading A?.B, it's equivalent to A && A.B except that A is only evaluated once, even if it's a complex expression like a proc call.

code/game/objects/items/stacks/stack.dm

@@ -58,7 +58,7 @@
 		new type(loc, max_amount, FALSE)
 	if(!merge_type)
 		merge_type = type
-	if(custom_materials && custom_materials.len)
+	if(custom_materials?.len)
 		mats_per_unit = list()
 		var/in_process_mat_list = custom_materials.Copy()
 		for(var/i in custom_materials)
@@ -242,7 +242,7 @@
 				O.setDir(usr.dir)
 			use(R.req_amount * multiplier)
 
-			if(R.applies_mats && custom_materials && custom_materials.len)
+			if(R.applies_mats && custom_materials?.len)
 				var/list/used_materials = list()
 				for(var/i in custom_materials)
 					used_materials[SSmaterials.GetMaterialRef(i)] = R.req_amount / R.res_amount * (MINERAL_MATERIAL_AMOUNT / custom_materials.len)