Reset custom language item value back to null

ref https://www.woltlab.com/community/thread/314393-the-phrase-filter-displays-phrases-with-custom-value-that-are-no-longer-customis/

Author: BurntimeX

wcfsetup/install/files/lib/acp/action/LanguageItemEditAction.class.php

@@ -70,6 +70,10 @@ public function handle(ServerRequestInterface $request): ResponseInterface
                 }
             }
 
+            if (!$data['languageUseCustomValue'] && !$languageItem->languageCustomItemValue) {
+                $data['languageCustomItemValue'] = null;
+            }
+
             $editor = new LanguageItemEditor($languageItem);
             $editor->update($data);