QGuiApplication::styleHints()->colorScheme() does not return a valid result

It seems that `QGuiApplication::styleHints()->colorScheme()` is not returning a valid result on Wayland.

I have confirmed that the desktop portal returns a valid value for colour preference with:

```

> gdbus call --session --dest org.freedesktop.portal.Desktop --object-path /org/freedesktop/portal/desktop --method org.freedesktop.portal.Settings.Read org.freedesktop.appearance color-scheme
(<<uint32 2>>,)

```

I originally reported this downstream on Telegram Desktop, because the application no longer respects the colour preference setting. The response that I received is that the behaviour that I'm seeing on Telegram Desktop is due to Qt not returning a valid value.

See: https://github.com/telegramdesktop/tdesktop/issues/27431