code: def test_missing_vals(self):
      m = self.missing.explore("continent")
      assert '"fillColor":null' in self._fetch_map_string(m)
  
      m = self.missing.explore("pop_est")
      assert '"fillColor":null' in self._fetch_map_string(m)
  
      m = self.missing.explore("pop_est", missing_kwds={"color": "red"})
      assert '"fillColor":"red"' in self._fetch_map_string(m)
  
      m = self.missing.explore(
          "continent", missing_kwds={"color": "red"}
      )
      assert '"fillColor":"red"' in self._fetch_map_string(m)
  
imports: import geopandas
  import numpy
  import pandas
  import pytest
  import shapely
  from packaging.version import Version
  from matplotlib import cm
  from matplotlib import colors
  from branca.colormap import StepColormap
label_desc: Use image node_id 21 for calling the TestExplore obj's underlying member method code with example usage: obj.test_missing_vals() without return types