[TASK] Avoid implicitly nullable class method parameter in `EXT:lowlevel`

With PHP 8.4 marking method parameter implicitly nullable
is deprecated and will emit a `E_DEPRECATED` warning. One
recommended way to resolve this, is making it explicitly
nullable using the `?` nullable operator or adding a null
tyype to an union type definition. [1]

This prepares the way towards PHP 8.4 compatibility.

[1] https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

Resolves: #104209
Releases: main
Change-Id: Iea94673b0d04d80f7b71f1cc9e936ccdbd7784e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84914


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