code: def test_null_values(dataframe):
      dataframe = dataframe.copy()
      dataframe.loc[1, "a"] = None
      dataframe.loc[4, "a"] = None
      dataframe.loc[3, "cities"] = None
      dataframe.loc[3, "Bell__Chart"] = None
      dataframe.loc[6, "decorated-elephant"] = None
  
      df = dataframe.data_description.df
      assert df.loc["a", "count"] == 7
      assert np.isclose(df.loc["a", "pct_missing"], 2 / 9)
  
      assert df.loc["cities", "count"] == 8
      assert np.isclose(df.loc["cities", "pct_missing"], 1 / 9)
  
      assert df.loc["Bell__Chart", "count"] == 8
      assert np.isclose(df.loc["Bell__Chart", "pct_missing"], 1 / 9)
  
      assert df.loc["decorated-elephant", "count"] == 8
      assert np.isclose(
          df.loc["decorated-elephant", "pct_missing"], 1 / 9
      )
  
file_path: pyjanitor/tests/functions/test_data_description.py
imports: import numpy
  import pytest
label_desc: Use image node_id 6 for calling a global function with example usage: test_null_values(dataframe) without return types