[TASK] Note nullable parameters and returns as such

Based on the code, these parameters are nullable. The doc
comments should reflect that. Doing so eliminates several
PHPStan warnings.

Whether these parameters ought to be nullable is a question
for another time.

Resolves: #97648
Releases: main, 11.5
Change-Id: Id0cc51d159ebb0321e94f7fb77a5d560950cfb05
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74791


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>