Vending Machine Launch Fix (#20215)

Fixes https://github.com/Aurorastation/Aurora.3/issues/20201

Fixes runtime in SSThrowing by having non-mobs default to targeting
chest
Refactors vendor reply code to be similar to how it handles ads and
slogans
Fixes interaction involving using a string to get the coin back from
vending machine

---------

Signed-off-by: Ben <91219575+Ben10083@users.noreply.github.com>
Co-authored-by: Ben10083 <Ben10083@users.noreply.github.com>
Co-authored-by: Fluffy <65877598+FluffyGhoster@users.noreply.github.com>
Co-authored-by: Wowzewow (Wezzy) <42310821+alsoandanswer@users.noreply.github.com>

code/modules/mining/coins.dm

@@ -13,7 +13,7 @@
 	slot_flags = SLOT_EARS
 	drop_sound = 'sound/items/drop/ring.ogg'
 	pickup_sound = 'sound/items/pickup/ring.ogg'
-	var/string_attached
+	var/string_attached = FALSE
 	var/sides = 2
 	var/cmineral = null
 	var/last_flip = 0 //Spam limiter
@@ -95,7 +95,7 @@
 		CC.amount = 1
 		CC.update_icon()
 		ClearOverlays()
-		string_attached = null
+		string_attached = FALSE
 		to_chat(user, SPAN_NOTICE("You detach the string from the coin."))
 	else ..()