[BUGFIX] Show error when copying a content element fails

A PHP 8.0 undefined array key warning was preventing
the flash message in the backend from appearing.
This is fixed by adding a fallback to null.

Resolves: #96132
Releases: main, 11.5
Change-Id: Id2f14a48e79ce1544ca325d025d5c428838ceb35
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72350


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>