[BUGFIX] Fix doctype annotations

According to https://docs.phpdoc.org/latest/guide/guides/types.html#union-types
it is only allowed to use "string|null" and not "?string"

Resolves: #96407
Releases: main, 11.5
Change-Id: I4e9ec53d05feb5cfffc4eebc29de447d6225ab66
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72749


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