[BUGFIX] Fix PHP 8 warning in BackendUtility

If fe_groups are deleted building the labels for the fe_groups
in titleAttribForPages will throw a PHP warning.

Resolves: #99712
Releases: main, 11.5
Change-Id: I61a340b74096c0da6eccc2eb0cd425c9fe02b71c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77578


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>