[FOLLOWUP][FEATURE] Allow Backend Logo to be set via extConf

The new PHP 7 syntax introduced also returns an empty
string, if set but empty.

Resolves: #74109
Releases: master
Change-Id: If902ea201cb49578ab2109b40de90c37dcb63b5d
Reviewed-on: https://review.typo3.org/47042


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>