message: Check for an empty map
  Summary: Check for an empty map because not all events have a json string in them
  Test Plan: run failing test
  Reviewers: jakeh, georgep
diff: @@ -51,6 +51,8 @@ class MappingTest(test_util.TensorFlowTestCase):
  for e in events:
  if e.type == IpuTraceEvent.COMPILE_END:
  + j = e.compile_end.tensor_map.decode('utf-8')
  + if len(j) > 0:
  tm = json.loads(e.compile_end.tensor_map.decode('utf-8'))
  bad_maps = []
  