Summary:
If the user list is empty (for example, because regular users are hidden
or sddm's configuration includes something like MinimumUid=2000)
show directly the username/password textboxes instead of an empty
user list.

Test Plan:
I edited /etc/sddm.conf and added MinimumUid=2000 to the [Users] group,
then restarted sddm.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3128