About Mann-Kendall test in Google Earth Engine

I would like to perform the Mann-Kendall test to attain the spatial change of my yearly growing season composite (named ‘growthdata), the’system:index’ below in the code is the corresponding year list, i.e., 2000,2001,2002,2003…, this is for differentiating the before and after images. I followed the official guidance from: https://developers.google.com/earth-engine/tutorials/community/nonparametric-trends this code works but I only get the image with all white color( with all 0 value). Can anyone tell me what’s the problem with my code?

    enter code here
    ////////////////////////////test mendall code////////////////////////////
var coll= growthdata.select('mean')
var afterFilter = ee.Filter.lessThan({
  leftField: 'system:index',
  rightField: 'system:index'
});
var sign = function(i, j) { // i and j are images
  return ee.Image(j).neq(i) // Zero case
      .multiply(ee.Image(j).subtract(i).clamp(-1, 1)).int();
};


var joined = ee.ImageCollection(ee.Join.saveAll('after').apply({
  primary: coll,
  secondary: coll,
  condition: afterFilter
}));
var sign = function(i, j) { // i and j are images
  return ee.Image(j).neq(i) // Zero case
    .multiply(ee.Image(j).subtract(i).clamp(-1, 1)).int()
}

var kendall = ee.ImageCollection(joined.map(function(current) {
  var afterCollection = ee.ImageCollection.fromImages(current.get('after'))
  return afterCollection.map(function(image) {
    // The unmask is to prevent accumulation of masked pixels that
    // result from the undefined case of when either current or image
    // is masked.  It won't affect the sum, since it's unmasked to zero.
    return ee.Image(sign(current, image)).unmask(0)
  })
  // Set parallelScale to avoid User memory limit exceeded.
}).flatten()).reduce('sum', 2)

var palette = ['red', 'white', 'green']
// Stretch this as necessary.
Map.addLayer(kendall, {min: -800, max: 200, palette: palette}, 'kendall')
print(joined)