[BUGFIX] Avoid warning in LanguageService

LanguageService->sL('LLL:') leads to a warning. Even
though this is of course a broken call, the code should
sanitize this case.

Resolves: #95636
Releases: master
Change-Id: Id96f4d1c7640cb042c5504a2dfbc2d04fbd9edc0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71634
Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Classes/Localization/LanguageService.php

@@ -182,7 +182,7 @@ public function sL($input)
             }
             $parts = explode(':', trim($restStr));
             $parts[0] = $extPrfx . $parts[0];
-            $output = $this->getLLL($parts[1], $this->readLLfile($parts[0]));
+            $output = $this->getLLL($parts[1] ?? '', $this->readLLfile($parts[0]));
         } else {
             // Use a constant non-localizable label
             $output = $input;