[BUGFIX] Enforce $GLOBALS['LANG']->sL() returns string

Calling LanguageService->sL() should *always* return
a string, even if it's empty. The patch enforces this.

This reveals a couple of incomplete unit test mocks,
which are fixed to keep the test suite happy.

Resolves: #95776
Releases: master
Change-Id: I2fbd272cebdbf76eeec354b3bc8750c172b3ece3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71974


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>