code: def test_should_be_serializable_when_fitted_without_slots(self):
      # Given
      features_factories = [
          {
              "factory_name": ShapeNgramFactory.name,
              "args": {"n": 1},
              "offsets": [0],
          },
          {
              "factory_name": IsDigitFactory.name,
              "args": {},
              "offsets": [-1, 0],
          },
      ]
      config = CRFSlotFillerConfig(
          tagging_scheme=TaggingScheme.BILOU,
          feature_factory_configs=features_factories,
      )
      dataset = {
          "language": "en",
          "intents": {
              "intent1": {
                  "utterances": [
                      {
                          "data": [
                              {
                                  "text": "This is an utterance without "
                                  "slots"
                              }
                          ]
                      }
                  ]
              }
          },
          "entities": {},
      }
  
      slot_filler = CRFSlotFiller(
          config, **self.get_shared_data(dataset)
      )
      slot_filler.fit(dataset, intent="intent1")
  
      # When
      slot_filler.persist(self.tmp_file_path)
  
      # Then
      metadata_path = self.tmp_file_path / "metadata.json"
      self.assertJsonContent(
          metadata_path, {"unit_name": "crf_slot_filler"}
      )
      self.assertIsNone(slot_filler.crf_model)
  
imports: from __future__ import unicode_literals
  import io
  import os
  from builtins import range
  from pathlib import Path
  from unittest import skipIf
  from mock import MagicMock, PropertyMock
  from sklearn_crfsuite import CRF
  from snips_nlu.constants import DATA, END, ENTITY, LANGUAGE_EN, SLOT_NAME, START, TEXT, RANDOM_STATE
  from snips_nlu.dataset import Dataset
  from snips_nlu.entity_parser import CustomEntityParserUsage
  from snips_nlu.exceptions import NotTrained
  from snips_nlu.pipeline.configs import CRFSlotFillerConfig
  from snips_nlu.preprocessing import tokenize, Token
  from snips_nlu.result import unresolved_slot
  from snips_nlu.slot_filler.crf_slot_filler import CRFSlotFiller, _ensure_safe, _encode_tag, CRF_MODEL_FILENAME
  from snips_nlu.slot_filler.crf_utils import TaggingScheme
  from snips_nlu.slot_filler.feature_factory import IsDigitFactory, NgramFactory, ShapeNgramFactory
  from snips_nlu.tests.utils import FixtureTest, TEST_PATH
label_desc: Use image node_id 18 for calling the TestCRFSlotFiller obj's underlying member method code with example usage: obj.test_should_be_serializable_when_fitted_without_slots() without return types