QGtk3Dialog::show(): add missing null-check to avoid warnings

Or else QObject::connect() warns about 'invalid null parameter' when
showing parentless dialogs.

Task-number: QTBUG-55298
Change-Id: I39b1dfc81e5da0c793c86cff763f946db15c13ae
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp

@@ -117,8 +117,10 @@ void QGtk3Dialog::exec()
 
 bool QGtk3Dialog::show(Qt::WindowFlags flags, Qt::WindowModality modality, QWindow *parent)
 {
-    connect(parent, &QWindow::destroyed, this, &QGtk3Dialog::onParentWindowDestroyed,
-            Qt::UniqueConnection);
+    if (parent) {
+        connect(parent, &QWindow::destroyed, this, &QGtk3Dialog::onParentWindowDestroyed,
+                Qt::UniqueConnection);
+    }
     setParent(parent);
     setFlags(flags);
     setModality(modality);