Fix initializing Postgres database

Summary:
The code to create the empty Postgres cluster on the first
start got accidentally lost in commit 180dc3e1.

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21716