[BUGFIX] Ensure that formatValue for dates/times returns a string

The unit tests are adjusted to have strict type checking.

The bug is actually covered with a test already, only the assertion
was wrong.

Resolves: #99521
Releases: main, 11.5
Change-Id: I52a1aa1a969bcc9d32df4ab6f43726bef671fe66
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77354


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>