code: def test_non_existent(self):
      self.assertIsNone(link_tf_api.tf_doc_url("tomhennigan"))
      self.assertIsNone(link_tf_api.tf_doc_url("autograph.1"))
  
file_path: sonnet/docs/ext/link_tf_api_test.py
imports: from absl.testing import absltest
  from docs.ext import link_tf_api
label_desc: Use image node_id 1 for calling the LinkTfApiTest obj's underlying member method code with example usage: obj.test_non_existent() without return types