message: Check both None and empty string.
  Sometime ${TPU_NAME} etc is not define, we receive "" as value instead of None.
diff: @@ -90,12 +90,12 @@ def main(unused_argv=None):
  tf_version = tf.__version__
  print('TensorFlow version %s detected' % tf_version)
  - if FLAGS.service_addr is None and FLAGS.tpu is None:
  + if not FLAGS.service_addr and not FLAGS.tpu:
  sys.exit('You must specify either --service_addr or --tpu.')
  tpu_cluster_resolver = None
  - if FLAGS.service_addr is not None:
  - if FLAGS.tpu is not None:
  + if FLAGS.service_addr:
  + if FLAGS.tpu:
  tf.logging.warn('Both --service_addr and --tpu are set. Ignoring '
  '--tpu and using --service_addr.')
  service_addr = FLAGS.service_addr
  