message: Allow the target of Transformer.visit_block to be None.
diff: @@ -325,6 +325,9 @@ class Base(gast.NodeTransformer):
  A list of AST node objects containing the transformed items fron nodes,
  except those nodes that have been relocated using after_visit.
  """
  + if nodes is None:
  + return None
  +
  results = []
  node_destination = results
  for node in nodes:
  