Fix saving "don't show again" when not using frameworksintegration

When not using frameworks integration we only get "false" in
shouldBeShownYesNo when the saved value is yes/no so set it to yes.

Also it was a bit weird before because we were using an uninitialized
value, so this is much better :)

BUGS: 409430

shell/shell.cpp

@@ -573,7 +573,7 @@ bool Shell::queryClose()
     if (m_tabs.count() > 1)
     {
         const QString dontAskAgainName = "ShowTabWarning";
-        KMessageBox::ButtonCode dummy;
+        KMessageBox::ButtonCode dummy = KMessageBox::Yes;
         if (shouldBeShownYesNo(dontAskAgainName, dummy))
         {
             QDialog *dialog = new QDialog(this);