[BUGFIX] Fix PHP 8 warning in ElementHistoryController

If $elementUid is 0 in ElementHistoryController::getEditLockFromElement
it must be handled seperatly since the call to BackendUtility::getRecord
will return null.

Resolves: #100000
Releases: main, 11.5
Change-Id: Ib53a5d06cbb098b9548a8293337b365d768d8fa3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77914


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>