Final test, fixes and parent calls.

code/game/objects/items/holy_weapons.dm

@@ -304,7 +304,7 @@
 	var/shield_icon = "shield-red"
 
 /obj/item/nullrod/staff/worn_overlays(isinhands, icon_file, used_state, style_flags = NONE)
-	. = list()
+	. = ..()
 	if(isinhands)
 		. += mutable_appearance('icons/effects/effects.dmi', shield_icon, MOB_LAYER + 0.01)
 

code/game/objects/items/tanks/watertank.dm

@@ -377,7 +377,7 @@
 		add_overlay(filling)
 
 /obj/item/reagent_containers/chemtank/worn_overlays(isinhands = FALSE, icon_file, used_state, style_flags = NONE) //apply chemcolor and level
-	. = list()
+	. = ..()
 	//inhands + reagent_filling
 	if(!isinhands && reagents.total_volume)
 		var/mutable_appearance/filling = mutable_appearance('icons/obj/reagentfillings.dmi', "backpackmob-10")

code/game/objects/items/tools/screwdriver.dm

@@ -54,7 +54,7 @@
 	. += base_overlay
 
 /obj/item/screwdriver/worn_overlays(isinhands = FALSE, icon_file, used_state, style_flags = NONE)
-	. = list()
+	. = ..()
 	if(isinhands && random_color)
 		var/mutable_appearance/M = mutable_appearance(icon_file, "screwdriver_head")
 		M.appearance_flags = RESET_COLOR