Fixes Compliance Issue With ListenBrainz API

[Menghini]

According to The Official ListenBrainz Documentation (page 89), the duration of a song is either duration and duration_ms, but not both. Maloja was only looking for duration and this caused issues with Navidrome as it reported duration_ms instead of duration so it would always be null. This would correct that issue.

[Menghini]

On second thought, in order to align more closely with Spotify request, I changed track_length to scrobble_duration which seems more likely to fit.
Example from Navidrome:

    {
      "time": 1733757481,
      "track": {
        "artists": [
          "Kazumi Totaka"
        ],
        "title": "Bees!",
        "album": {
          "artists": [
            "Kazumi Totaka"
          ],
          "albumtitle": "Animal Crossing"
        },
        "length": null
      },
      "duration": 38560,
      "origin": "client:navidrome"
    },

Example from Spotify:

    {
      "time": 1733744505,
      "track": {
        "artists": [
          "Ethan Slater",
          "Marissa Bode",
          "Ariana Grande",
          "Cynthia Erivo",
          "Jonathan Bailey"
        ],
        "title": "Dancing Through Life",
        "album": {
          "artists": [
            "Ariana Grande",
            "Cynthia Erivo",
            "Wicked Movie Cast"
          ],
          "albumtitle": "Wicked: The Soundtrack"
        },
        "length": 587
      },
      "duration": 586,
      "origin": "client:spotify"
    },