code: def test_index_field_empty_field_works(self):
      index_field = IndexField(4, self.text)
      empty_index = index_field.empty_field()
      assert empty_index.sequence_index == -1
  
file_path: allennlp/tests/data/fields/index_field_test.py
imports: import numpy
  import pytest
  from allennlp.common.checks import ConfigurationError
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.data import Token
  from allennlp.data.fields import TextField, IndexField
  from allennlp.data.token_indexers import SingleIdTokenIndexer
label_desc: Use image node_id 4 for calling the TestIndexField obj's underlying member method code with example usage: obj.test_index_field_empty_field_works() without return types