Fix non standard wizard for default theme

Somehow a line "PreferredStyles =" is interpreted by QSettings
as a QStringList with a empty string.

Simply remove empty strings on reading the settings, as no
style can have that name.

Change-Id: If544e55f759c201d0621c71424a4ebedc8456d61
Task-number: QTCREATORBUG-15043
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>

src/libs/utils/theme/theme.cpp

@@ -266,6 +266,7 @@ void Theme::readSettings(QSettings &settings)
     {
         d->name = settings.value(QLatin1String("ThemeName"), QLatin1String("unnamed")).toString();
         d->preferredStyles = settings.value(QLatin1String("PreferredStyles")).toStringList();
+        d->preferredStyles.removeAll(QLatin1String(""));
     }
     {
         settings.beginGroup(QLatin1String("Palette"));