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/layoutbuilder.cpp

@@ -511,6 +511,7 @@ LayoutBuilder::Setter title(const QString &title, BoolAspect *checker)
 
 LayoutBuilder::Break br;
 LayoutBuilder::Stretch st;
+LayoutBuilder::Space empty(0);
 
 } // Layouting
 } // Utils