[BUGFIX] Fix type annotation of HistoryService::differencesObject

This field actually is nullable and needs to be annotated accordingly.

This change fixes a PHPStan warning.

Resolves: #97385
Releases: main, 11.5
Change-Id: If64d64665e0ffd9419d51fcda2b84befa8ebceef
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74297


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