[BUGFIX] Do not iterate over empty logger configuration

Fixes PHP notices thrown by accessing non existing array key.

Resolves: #83004
Releases: master
Change-Id: Ib5ff0ace0c9eb9a9987693176c4bee25026e2eb8
Reviewed-on: https://review.typo3.org/54653


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>