message: Change _enable_get_next_as_optional to False by default.
diff: @@ -125,7 +125,7 @@ class InputIteratorImpl(InputIterator):
  def __init__(self, input_workers, iterators, **kwargs):
  # TODO(b/128995245): Remove this temporary flag once the zero batch case can
  # be correctly handled.
  - self._enable_get_next_as_optional = True
  + self._enable_get_next_as_optional = False
  if len(kwargs) > 1:
  raise ValueError("InputIteratorImpl constructor only takes one "
  "experimental flag now")
  