Track available markets sometimes empty list instead of missing

[felix-hilden]

Hello,

The `available_markets` attribute of a track seems to vary from missing to an empty list. This happens in the track call, when a market is specified. Previously I think the markets were always missing. It seems that often on first attempt the attribute is missing, but subsequently returned as an empty list. Not always though.

Any chance this is not intended? It makes validating the response just a tad more complicated.

Example track: "spotify:track:6v3KW9xbzN5yKLt9YKDYA2" with "US" market

[felix-hilden]

It also seems that this is not the case for the "tracks" endpoint, only for a single track.