Controller/Adminhtml/FastlyCdn/BasicAuthentication/CheckAuthDictionary.php
public function execute()
    {
        $result = $this->resultJsonFactory->create();
        try {
            $activeVersion = $this->getRequest()->getParam('active_version');

            $dictionaryName = Config::AUTH_DICTIONARY_NAME;
            $dictionary = $this->api->getSingleDictionary($activeVersion, $dictionaryName);

            if ((is_array($dictionary) && empty($dictionary)) || $dictionary == false) {
                return $result->setData(['status' => false]);
            } else {
                return $result->setData(['status' => true]);
            }
        } catch (\Exception $e) {
            return $result->setData([
                'status'    => false,
                'msg'       => $e->getMessage()
            ]);
        }
    }
public function execute()
    {
        $result = $this->resultJsonFactory->create();
        try {
            $activeVersion = $this->getRequest()->getParam('active_version');

            $dictionaryName = Config::AUTH_DICTIONARY_NAME;
            $dictionary = $this->api->getSingleDictionary($activeVersion, $dictionaryName);

            if ((is_array($dictionary) && empty($dictionary)) || $dictionary == false) {
                return $result->setData(['status' => false]);
            } else {
                return $result->setData(['status' => true]);
            }
        } catch (\Exception $e) {
            return $result->setData([
                'status'    => false,
                'msg'       => $e->getMessage()
            ]);
        }
    }
Check if AUTH dictionary exists

@return \Magento\Framework\Controller\Result\Json