PlaylistTrackObject.track can be null

[ghost]

example playlist: spotify:playlist:6fzw3GiLktfRKczD6sl6mT
bug: spotify/web-api#958

Traceback:
File "spotipy/model/playlist.py", line 33, in post_init
self.items = [PlaylistTrack(**t) for t in self.items]
File "spotipy/model/playlist.py", line 33, in
self.items = [PlaylistTrack(**t) for t in self.items]
File "", line 8, in init
File "spotipy/model/playlist.py", line 24, in post_init
self.track = FullTrack(**self.track)
TypeError: type object argument after ** must be a mapping, not NoneType

[ghost]

This is actually documented feature:
"Information about the tracks of the playlist. Note, a track object may be null. This can happen if a track is no longer available."
https://developer.spotify.com/documentation/web-api/reference/playlists/get-playlist/

[felix-hilden]

Reimplement this as a check on the track level as discussed in the PR review.