Another bunch of fixes for TGUI inputs (I'll be cursed...) (#25009)

* Oh god, oh fuck, WHY DIDN'T I USE isnull()???

* More correct checks

* That's all

* That's all x2

* Flavor fix

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

@@ -293,7 +293,7 @@
 	var/min = 0
 	var/max = get_amount()
 	var/stackmaterial = tgui_input_number(user, "How many sheets do you wish to take out of this stack? (Max: [max])", "Stack Split", max_value = max)
-	if(stackmaterial == null || stackmaterial <= min || stackmaterial > get_amount())
+	if(isnull(stackmaterial) || stackmaterial <= min || stackmaterial > get_amount())
 		return
 	if(!Adjacent(user, 1))
 		return

code/game/objects/items/weapons/AI_modules.dm

@@ -215,7 +215,7 @@ AI MODULES
 /obj/item/aiModule/freeform/attack_self(mob/user as mob)
 	..()
 	var/new_lawpos = tgui_input_number(user, "Please enter the priority for your new law. Can only write to law sectors 15 and above.", "Law Priority", lawpos, MAX_SUPPLIED_LAW_NUMBER, MIN_SUPPLIED_LAW_NUMBER)
-	if(!new_lawpos || new_lawpos == lawpos)
+	if(isnull(new_lawpos) || new_lawpos == lawpos)
 		return
 	lawpos = new_lawpos
 

code/game/objects/items/weapons/cards_ids.dm

@@ -499,7 +499,7 @@
 							var/mob/living/carbon/human/H = user
 							default = H.age
 						var/new_age = tgui_input_number(user, "What age would you like to be written on this card?", "Agent Card Age", default, 300, 17)
-						if(!Adjacent(user) || !new_age)
+						if(!Adjacent(user) || isnull(new_age))
 							return
 						age = new_age
 						to_chat(user, "<span class='notice'>Age changed to [new_age].</span>")
@@ -535,7 +535,7 @@
 
 					if("Money Account")
 						var/new_account = tgui_input_number(user, "What money account would you like to link to this card?", "Agent Card Account", 12345, max_value = 9999999)
-						if(!Adjacent(user) || !new_account)
+						if(!Adjacent(user) || isnull(new_account))
 							return
 						associated_account_number = new_account
 						to_chat(user, "<span class='notice'>Linked money account changed to [new_account].</span>")

code/game/objects/items/weapons/picket_signs.dm

@@ -15,7 +15,7 @@
 /obj/item/picket_sign/attackby(obj/item/W, mob/user, params)
 	if(is_pen(W) || istype(W, /obj/item/toy/crayon))
 		var/txt = tgui_input_text(user, "What would you like to write on the sign?", "Sign Label", max_length = 30)
-		if(!txt)
+		if(isnull(txt))
 			return
 		label = txt
 		src.name = "[label] sign"