[BUGFIX] PHP warning while configuring extension

Change-Id: I6291931e37d6d86babeb151a162aa9203ccfdc6e
Reviewed-on: https://review.typo3.org/29122
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers