wallpapers/image: Fix when used outside plasmashell
When used outside plasmashell, like in the kscreenlocker KCM, the Plasmoid attached property isn't atcually undefined but just an empty object. Now we check for that case properly.
When used outside plasmashell, like in the kscreenlocker KCM, the Plasmoid attached property isn't atcually undefined but just an empty object. Now we check for that case properly.