wallpapers/image: fall back to default wallpaper when url is empty

It's expected to use the default wallpapaer when url is empty, which is
also the default value.

When closing the wallpaper config dialog without saving config, the
source property of MediaProxy will become empty if the previous
wallpaper is the default wallpaper, but in setSource empty url will
be directly rejected instead of switching to the default wallpaper.

This fixes the wrong behavior by calling `useSingleImageDefaults` when
url is empty, and reset the source url in `useSingleImageDefaults` so it
can find a wallpaper from the current Plasma theme.

BUG: 460692