[TASK] Improve return type annotations for GU::trimExplode

This particularly helps PHPStan in extensions that call those methods
detect unnecessary checks for empty strings if
`GU::trimExplode` is called with `$removeEmptyValues = true`.

Used command:

> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #101395
Related: #99147
Releases: main, 12.4, 11.5
Change-Id: I6945cc0698b0777a05cb9327b342aa9aa7dee098
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80138


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>