Utils: Introduce a Layouting::empty item

A bit more explicit than the current mix of Space() and asserting {}.

Change-Id: Ia296b1d23294dfccd1e33827e136b821a4d2c6fe
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/aspects.cpp

@@ -1491,7 +1491,7 @@ void SelectionAspect::addToLayout(LayoutBuilder &builder)
             button->setChecked(i == value());
             button->setEnabled(option.enabled);
             button->setToolTip(option.tooltip);
-            builder.addItems({{}, button});
+            builder.addItems({Layouting::empty, button});
             d->m_buttons.append(button);
             d->m_buttonGroup->addButton(button, i);
             if (isAutoApply()) {