[BUGFIX] Possible PHP warning with AdditionalConfiguration.php

Configuration file AdditionalConfiguration.php may not be present and
causes a PHP warning when opening Backend configuration module.

Change-Id: Ic0a3c6da644f65ae0e8f746fd07a12ced4c06a56
Fixes: #54633
Reviewed-on: https://review.typo3.org/26600
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers