Z-Prepass doesn't render correctly if scene has only transparent objects with depth write

Found while investigating this: https://bugreports.qt.io/browse/QTBUG-95337

When the scene has only transparent objects with some of them enabling depth write, and scene has z-prepass enabled, those objects are not rendered correctly. The depth write doesn't affect the rendering result and the scene looks as if it is not enabled at all. If then one adds opaque objects to the scene, the transparent obects are rendered correctly.