mpris: Work-around some players not handling empty TrackId
It's unclear what to pass to the SetPosition()
method when the
application doesn't implement the TrackList
interface and so doesn't
have TrackId
metadata.
Export placeholder /org/mpris/MediaPlayer2/TrackList/NoTrack
in the
meanwhile, which we'll ignore in when SetPosition()
is called.
See https://gitlab.freedesktop.org/mpris/mpris-spec/-/issues/19
Closes: #538 (closed)