Fix issue when there is no WidgetStyle value in kdeglobals

Summary:
KConfig XT does not store default values in conf files. KCM Style applying default style, KHintsSetting expect a value in kdeglobals or ignore the notify change.
fix to provide a default value to allow applying default widget style.

Reviewers: Plasma, ervin, bport, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24918