[BUGFIX] Always call GU::intExplode with a string in EXT:frontend

This helps avoid breakage when GeneralUtility will switch to
strict mode.

This change also prevents a bunch of possible invalid
array access warnings.

Change-Id: I64a0d5a5fce7bc3e3eb0aec97ebfbaa57675d3b8
Resolves: #97584
Related: #97578
Releases: main, 11.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76012


Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>