Show useful error message when attempting to finish a transaction with no items

action.php

@@ -38,6 +38,11 @@ switch ($VARS['action']) {
         $database->action(function ($database) {
             global $VARS, $binstack, $error, $oktx;
 
+            if (empty($VARS['items'])) {
+                $error = lang("no items", false);
+                return false;
+            }
+
             $items = $VARS['items'];
             $payments = $VARS['payments'];
             $customer = $VARS['customer'];

lang/en_us.php

@@ -137,4 +137,5 @@ define("STRINGS", [
     "card x added" => "Gift card #{arg} added.",
     "card x saved" => "Gift card #{arg} updated.",
     "open drawer" => "Open Drawer",
+    "no items" => "No items in transaction.",
 ]);