For albums that are listed with a non-"main" artist first (ex: 37691520), the album artist replacement was returning an empty string. This isn't a problem with the default settings, but if you format albums into `{album_artist}/...`, the path becomes `/...` in this case, and is interpreted as an absolute path instead of a relative one, causing the download to fail.
This changes the logic so that if the `first_only` flag is passed, it will return after the first artist is actually added to the list, no longer returning an empty string.
