code: def test_empty_span_field_works(self):
      span_field = SpanField(1, 3, self.text)
      empty_span = span_field.empty_field()
      assert empty_span.span_start == -1
      assert empty_span.span_end == -1
  
file_path: allennlp/tests/data/fields/span_field_test.py
imports: import numpy
  import pytest
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.data import Token
  from allennlp.data.fields import TextField, SpanField
  from allennlp.data.token_indexers import SingleIdTokenIndexer
label_desc: Use image node_id 6 for calling the TestSpanField obj's underlying member method code with example usage: obj.test_empty_span_field_works() without return types