Controller/Adminhtml/FastlyCdn/BasicAuthentication/CheckAuthUsersAvailable.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)) || $dictionary == false || !isset($dictionary->id)) {
                return $result->setData([
                    'status'    => 'empty',
                    'msg'       => 'Basic Authentication cannot be enabled because there are no users assigned to it.'
                ]);
            } else {
                $authItems = $this->api->dictionaryItemsList($dictionary->id);

                if (is_array($authItems) && empty($authItems)) {
                    return $result->setData([
                        'status' => 'empty',
                        'msg'    => 'Basic Authentication cannot be enabled because there are no users assigned to it.'
                    ]);
                }

                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)) || $dictionary == false || !isset($dictionary->id)) {
                return $result->setData([
                    'status'    => 'empty',
                    'msg'       => 'Basic Authentication cannot be enabled because there are no users assigned to it.'
                ]);
            } else {
                $authItems = $this->api->dictionaryItemsList($dictionary->id);

                if (is_array($authItems) && empty($authItems)) {
                    return $result->setData([
                        'status' => 'empty',
                        'msg'    => 'Basic Authentication cannot be enabled because there are no users assigned to it.'
                    ]);
                }

                return $result->setData(['status' => true]);
            }
        } catch (\Exception $e) {
            return $result->setData([
                'status'    => false,
                'msg'       => $e->getMessage()
            ]);
        }
    }
Check if authentication users are available

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