I modified nlmpris.cpp from line 151 to 161 to:
If the file is playing doesn't have metadata, then m_artist must show " " and not %artist.
The same for m_track...
But if your mp3 file, doesn't have metadata, it show the name of file without mp3 extension

This is my piece of code

    if ( newTrack != m_track )
    {
        m_newTrack = true;
        m_track = newTrack;
        if (m_track.isEmpty())
        {
            m_track = " ";
        }
        // Fetch album
        m_album = metaData["album"].toString();
        if (m_album.isEmpty())
        {
            m_album = " ";
        }
        // Fetch artist
        m_artist = metaData["artist"].toString();
        if (m_artist.isEmpty())
        {
            m_artist = " ";
            if (m_track.isEmpty())
            {
                QStringList l;
                const QString tracktemp = metaData["location"].toString();
                l = tracktemp.split("/", QString::SkipEmptyParts);
                m_track = l[l.count() - 1];
            }
        }
    }

What do you think?

PD: It's my first time with QT, i'm a "rookie" :D

Reproducible: Always