[BUGFIX] Fix undefined key warning in EXT:install/mail preset with PHP8

This fix a warning when you click on "SMTP Settings" in "Configuration preset".

Resolves: #96348
Releases: main, 11.5
Change-Id: Ic6ec6b7efd9dde2f7c365fa391e6c358192dc23b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72644


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>