[BUGFIX] Correct type annotation of GU::callUserFunction()

The argument `$funcName` is now annotated as `non-empty-string|\Closure`
to reflect the actual allowed input types. Additionally corresponding
test in `GeneralUtilityTest.php` is also annotated properly.

Used command:

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #97568
Releases: main, 11.5
Change-Id: I7ce640d3565f099e6dc5ba62c59e5616dc4dc6e4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74546


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>