XdgUserDirs: Return a fallback when the path is empty
XdgUserDirs: Return a fallback when the path is empty

If the path to the folder is empty we return the default value.
It's what xdg-user-dirs also do.

lxde/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg,luis-pereira/libqtxdg,libqtxdg/libqtxdg,libqtxdg/libqtxdg