[BUGFIX] Fix array access warnings in EditDocumentController

$this->firstEl is checked for !== null several times in
EditDocumentController, but it most certainly isn't, because
reset() returns false instead of null, if the array is empty.

This patch resolves this problem by setting $this->firstEl to
null, if reset($this->elementsData) returned false.

Resolves: #96342
Releases: main, 11.5
Change-Id: I45b7aff02d7b601e16fd05862542a9e3955de908
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72638


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