code: def test_null(self):
      x = pd.Series([np.nan, pd.NA, None, "@test"])
  
      actual = self.primitive().get_function()(x)
      expected = [np.nan, np.nan, np.nan, 1.0]
      np.testing.assert_array_equal(actual, expected)
  
file_path: featuretools/featuretools/tests/primitive_tests/natural_language_primitives_tests/test_number_of_mentions.py
imports: import numpy
  import pandas
  from featuretools.primitives import NumberOfMentions
  from featuretools.tests.primitive_tests.utils import PrimitiveTestBase, find_applicable_primitives, valid_dfs
label_desc: Use image node_id 4 for calling the TestNumberOfMentions obj's underlying member method code with example usage: obj.test_null() without return types