fix(install): fatal exception during installation

During the installation process 'normal' pages are drawn. Part if the
page is the metatag for sitedescription. This isn't available in $CONFIG
so the system goes to the database, which is unavailable.

fixes issue caused by 66f06919735e3de97b8262cc13c7044df755795b
Merge pull request #6488 from jeabakker/fatal-exception-during-installation

fix(install): fatal exception during installation