[TASK] Update TYPO3 Constant + guards

Configuration/TCA/Overrides/sys_template.php

 <?php
-
+defined('TYPO3') or die();
 \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addStaticFile(
 	'sg_youtube',
 	'Configuration/TypoScript',

Configuration/TCA/Overrides/tt_content.php

 <?php
-
+defined('TYPO3') or die();
 $GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist']['sgyoutube_youtube'] = 'pi_flexform';
 \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
 	'sgyoutube_youtube',

ext_localconf.php

@@ -24,9 +24,7 @@
  *  This copyright notice MUST APPEAR in all copies of the script!
  ***************************************************************/
 
-if (!defined('TYPO3')) {
-	die('Access denied.');
-}
+defined('TYPO3') or die();
 
 \TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
 	'SgYoutube',