post release 1.2 fixes (#1068)

* fix release command
* fix permission name in UPGRADING
* added missing korean calendar translations
* fixed entry.total for invoice with fixed rates

src/Invoice/InvoiceItem.php

@@ -64,7 +64,7 @@ class InvoiceItem
      */
     private $additionalFields = [];
 
-    public function addAdditionalField(string $name, string $value): InvoiceItem
+    public function addAdditionalField(string $name, ?string $value): InvoiceItem
     {
         $this->additionalFields[$name] = $value;