I cleaned the array using:

    const mplayerArgumentsArrayFilter = mplayerArgumentsArray.filter(discardEmptyArgument);
    function discardEmptyArgument(value, index, array) {
      return value != '';
    }

Then spawned using that array:

    // Spawn a new mplayer process
    const env = { ...process.env, DISPLAY: ':0' };
    const mplayerProcess = spawn(`mplayer`, mplayerArgumentsArrayFilter, {env: env});

Now it is working correct.

When using

    // Spawn a new mplayer process
    const env = { ...process.env, DISPLAY: ':0' };
    const mplayerProcess = spawn(`mplayer`,
       ['-rtsp-stream-over-tcp',
        '-noborder',
        '-monitoraspect', '0',
        '-vf', 'rotate=-1',
        '-geometry', '5:225',
        '-x', '640',
        '-y', '360',
        'rtsp://axisviewer:password@192.168.178.55/axis-media/media.amp?streamprofile=ACC_Low'],
        {env: env});

The stream is correct on the monitor.

When using

    // Spawn a new mplayer process
    const env = { ...process.env, DISPLAY: ':0' };
    const mplayerProcess = spawn(`mplayer`,
       ['',
        '', '',
        '-rtsp-stream-over-tcp',
        '',
        '', '',
        '-noborder',
        '-monitoraspect', '0',
        '-vf', 'rotate=-1',
        '-geometry', '5:225',
        '-x', '640',
        '-y', '360',
        'rtsp://axisviewer:password@192.168.178.55/axis-media/media.amp?streamprofile=ACC_Low'],
        {env: env});

The stream is NOT correct on the monitor (MPlayer window in the center).

So the problem is spawning with empty parameters.
When running it from the command line, this is not an issue, as the shell takes care of the empty places between the arguments.

I tried with undefined and null iso ''but that does not work.

Only idea how to solve this ?
Cleaning the array somehow ?