code: def test_all_nans(self, na_value):
      x = pd.Series([na_value, na_value, na_value])
      primitive_func = self.primitive().get_function()
      answers = pd.Series([np.nan, np.nan, np.nan])
      pd.testing.assert_series_equal(
          primitive_func(x), answers, check_names=False
      )
  
file_path: featuretools/featuretools/tests/primitive_tests/natural_language_primitives_tests/test_mean_characters_per_word.py
imports: import numpy
  import pandas
  import pytest
  from featuretools.primitives import MeanCharactersPerWord
  from featuretools.tests.primitive_tests.utils import PrimitiveTestBase, find_applicable_primitives, valid_dfs
label_desc: Use image node_id 5 for calling the TestMeanCharactersPerWord obj's underlying member method code with example usage: obj.test_all_nans(na_value) without return types