message: Don't annotate _tflite_function_uuid when there's no OpHints
diff: @@ -728,10 +728,10 @@ def _find_all_hints_in_nodes(nodes):
  for node in nodes:
  attr = node.attr
  # This is an op hint if it has a FUNCTION_UUID_ATTR, otherwise skip
  - uuid = attr[OpHint.FUNCTION_UUID_ATTR].s
  if (OpHint.FUNCTION_UUID_ATTR not in attr
  or not attr[OpHint.FUNCTION_UUID_ATTR].s):
  continue
  + uuid = attr[OpHint.FUNCTION_UUID_ATTR].s
  # Start building function
  call_def = func_calls[uuid]
  