message: Update check for `None` in base_layer_utils.
  Fixes broken test tensorflow_probability/python/layers:variable_input_test
diff: @@ -589,7 +589,7 @@ def mark_as_return(outputs, acd):
  # Handle TensorFlow Probability attached metadata.
  # TODO(b/132076537): Remove this once TFP uses `CompositeTensor`.
  - if getattr(tensor, '_tfp_distribution', None):
  + if getattr(tensor, '_tfp_distribution', None) is not None:
  return_tensor._tfp_distribution = tensor._tfp_distribution
  return return_tensor
  