code: def testGetSourceWriter_noWriter(self):
      actual_writer = event_util.GetSourceWriter(
          event_pb2.SourceMetadata(writer="")
      )
      self.assertIsNone(actual_writer)
  
file_path: tensorboard/tensorboard/backend/event_processing/event_util_test.py
imports: from unittest import mock
  from tensorboard import test
  from tensorboard.backend.event_processing import event_util
  from tensorboard.compat.proto import event_pb2
  from tensorboard.util import tb_logging
label_desc: Use image node_id 4 for calling the EventUtilTest obj's underlying member method code with example usage: obj.testGetSourceWriter_noWriter() without return types