[BUGFIX] Fix PHP 8 warning in TypoScriptFrontendController

Added Null coalescing operator, when accessing
$this->register['SYS_LASTCHANGED'].

Resolves: #99876
Releases: main, 11.5
Change-Id: I56a94d20195fdb82000259fdbe3e2eb723695383
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77819


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>