[BUGFIX] Update the extConf check to check for array after unserializing

Classes/Utility/ExtensionUtility.php

@@ -46,7 +46,7 @@ class ExtensionUtility {
 			$extConf = \unserialize(
 				$GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['sg_mail'], ['allowed_classes' => FALSE]
 			);
-			return $extConf ?? [];
+			return is_array($extConf) ? $extConf : [];
 		}
 
 		return $GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS']['sg_mail'] ?? [];