[!!!][TASK] Ensure continuous array keys for GU::intExplode() return

To follow the principle of the "smallest surprise" concerning the
behavior of our code, the integer array keys of the return values of
`GU::intExplode()` should be ensured to be gap-free.

Resolves: #99898
Releases: main
Change-Id: I683ee805e2ad4efd2fd182f4f94b7f95fa6cf2be
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77802


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>