[BUGFIX] Fix check for new global extension configuration option

Classes/Service/RegisterService.php

@@ -84,10 +84,10 @@ class RegisterService implements \TYPO3\CMS\Core\SingletonInterface {
 	private function registerExtensions(): array {
 		$this->registerArray = [];
 
-		$extensionConfiguration = unserialize($GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['sg_mail'], []);
+		$extensionConfiguration = unserialize($GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['sg_mail'], ['array']);
 
 		// @TODO remove in version 5.0.0
-		if ($extensionConfiguration['enableSecurity'] === 0) {
+		if (((int) $extensionConfiguration['enableSecurity']) === 0) {
 			MailTemplateService::registerExtensions();
 		}