get_album: fix invalid track list artists (#617)

[czifumasa]

Fixed invalid artists info for get_album from #617 issue.

Bug was a result of my changes in #612.

This is a flexColumn for an artist:

{
"musicResponsiveListItemFlexColumnRenderer": {
  "text": {},
 "displayPriority": "MUSIC_RESPONSIVE_LIST_ITEM_COLUMN_DISPLAY_PRIORITY_HIGH"
 }
}

it was skipped in parsing, because it did not have navigationEndpoint and text was empty. "Plays" column has been used as an artist instead. I removed empty text check. Now it will be used as unrecognized artist column. Then parse_song_artists method will return None and in the end in get_album method if artist track is None, then primary album artist will be used. So it will work exactly as in 1.7.3 and before.

Closes #617.

[sigma67]

@MarvinSchenkel can you please review?

[MarvinSchenkel]

@MarvinSchenkel can you please review?

Yep all good, just tested with the feature branch of this PR.