code: def test_gradient_default_none(self):
      layer = _HeatmapOptionsMixin()
      assert layer.gradient is None
  
imports: import unittest
  import pytest
  import traitlets
  from ..heatmap import _HeatmapOptionsMixin, heatmap_layer, Heatmap, WeightedHeatmap
  from ..geotraitlets import InvalidPointException, InvalidWeightException
label_desc: Use image node_id 1 for calling the TestHeatmapOptionsMixin obj's underlying member method code with example usage: obj.test_gradient_default_none() without return types