Null pointer dereference in QHighDpiScaling::updateHighDpiScaling if QGuiApplication::screens is empty

If a QGuiApplication is started with no display connected or all connected displays turned off the eglfs qpa will not create any QScreen instances.

init_platform in qguiapplication.cpp correctly guards its call to QHighDpiScaling::updateHighDpiScaling() but QGuiApplicationPrivate::eventDispatcherReady() calls it unconditionally.

QHighDpiScaling::updateHighDpiScaling() calls QGuiApplication::primaryScreen() but doesn't account for the fact that it will return a null pointer if no QScreens have been created. The application will then crash when it tries to dereference the null pointer.