[TASK] Avoid reading languagefiles if argument is empty

'LanguageService->sL()' could be called with an empty
input string, thus reading language files and caching
the result is superflous.

This patch adds an early check and return to 'sL()'
to mitigate that.

Resolves: #96666
Releases: main, 11.5
Change-Id: Ib97958be6f4ffbd96cdfc158c9307f5007d31f2f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73163


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>