document that in edge cases, playlist tracks may be null

Author: adamint

src/main/kotlin/com/adamratzman/spotify/models/Playlist.kt

@@ -62,7 +62,7 @@ data class SimplePlaylist(
  * @property addedAt The date and time the track was added. Note that some very old playlists may return null in this field.
  * @property addedBy The Spotify user who added the track. Note that some very old playlists may return null in this field.
  * @property isLocal Whether this track is a local file or not.
- * @property track Information about the track.
+ * @property track Information about the track. In rare occasions, this field may be null if this track's API entry is broken.
  */
 data class PlaylistTrack(
     @Json(name = "primary_color") val primaryColor: String? = null,