Track preview_url can be null

[johnelse]

This can be handled by using the nullable atdgen type.

Repro:

utop # Spotify_web_api.Search.search_tracks "imagine dragons radioactive";;
Exception:                                                                                                             
Yojson.Json_error "Line 846, bytes 24-56:\nExpected '\"' but found 'null,\n      \"track_number\" : 1,\n  '".