[BUGFIX] Allow integer keys in ArrayUtility::flatten()

Instead of breaking the execution with a TypeError if
an array to be flattened by ArrayUtility::flatten()
has integer keys, this change allows the method to
handle this case correctly.

Resolves: #101208
Related: #97202
Releases: main, 12.4
Change-Id: I05d47da80e088f024536fe15f5f2c2802c22a921
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79569


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>