Show correct remaining stock for Gambleshops

src/main/java/com/alttd/playershops/shop/PlayerShop.java

@@ -89,7 +89,14 @@ public class PlayerShop {
                 return (int) (balance / getPrice());
             }
             case GAMBLE -> {
-                return getInventory().getContents().length;
+                ArrayList<ItemStack> contents = new ArrayList<>();
+                for (ItemStack it : this.getInventory().getContents()) {
+                    if (it != null) {
+                        contents.add(it);
+                    }
+
+                }
+                return contents.size();
             }
             default -> {
                 return 0;