If the theme set in the SDDM config is missing, select first possible entry

Summary:
It's possible for the SDDM config to not list a theme, particularly if you're using
the default theme.

Test Plan: Set a blank SDDM config. Opened the KCM, it looked sane.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2876