code: def test_ner(self):
      sent = "今天天气很好。"
      test_field = CnTextField(sent)
      self.assertEqual(
          ([], ["O", "O", "O", "O", "O", "O", "O"]), test_field.ner()
      )
      sent = ""
      test_field = CnTextField(sent)
      self.assertEqual(([], []), test_field.ner())
  
imports: import unittest
  from textflint.input.component.field.cn_text_field import *
label_desc: Use image node_id 6 for calling the TestCnTextField obj's underlying member method code with example usage: obj.test_ner() without return types