Fix installer

Configuration from extension.json is not loaded by the installer. Hence
there is no 'ReadingLists' configuration yet. Detect that edge case and
set a default value for 'centralWiki' configuration parameter.

Bug: T196567
Change-Id: Icf707e60069070cebcda2906985d3960b3c555ec
(cherry picked from commit 27ed0cedae81542aaf4946a229286d6adb9a679e)