[kcmkwin/kwindecoration] Fix save of theme name

Summary:
If a user saves a decoration that has a plugin but no theme we currently
delete the entry saving just the plugin/library.

This is an issue as on load we call:

    const QString plugin = config.readEntry("library", s_defaultPlugin);
    const QString theme = config.readEntry("theme", s_defaultTheme);

That would give us a non-default library with the default theme name,
which would be invalid.

If compiled with Breeze s_defaultTheme is blank so this ends up being
effectively the same thing. In the normal case it will neither break
nor fix anything.

Test Plan:
None.
Just happened to see it when doing a code review.

Reviewers: #kwin, vpilo

Reviewed By: vpilo

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18146