Fix multiplayer-only crash from Amber Acorn being the only joker

manifest.json

@@ -3,7 +3,7 @@
     "id": "Roland",
     "name": "Roland",
     "prefix": "Roland",
-    "version": "2.8.9",
+    "version": "2.8.10",
     "badge_colour": "8BE9FD",
     "display_name": "Roland",
     "main_file": "src/main.lua",

src/joker.lua

@@ -665,6 +665,11 @@ joker {
         card.ability.extra.context = context
         local cards = card.area.cards
         local keys = f(cards):where(f().nq(card)):keys():table()
+
+        if not next(keys) then
+            return
+        end
+
         local key = pseudorandom_element(keys, pseudoseed "Roland_amber") or card.rank
         local next = card.rank < key and 1 or -1