PXD-2468 ⁃ Investigate sheepdog export error

[philloooo]

the sheepdog tests in gen3-qa submit file invalid property and update file with invalid property fail with this error in the sheepdog logs:

128.135.98.36 - uid:2,[email protected] [07/Dec/2018:20:44:51 +0000] "PUT /v0/submission/jnkns/jenkins HTTP/1.0" 400 833 "-" "-" 0.069 0.068
ERROR:root:Rolling back session [404] - Unable to find null
[2018-12-07 20:44:51,446] ERROR in api: [404] - Unable to find null
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/sheepdog/sheepdog/auth/__init__.py", line 122, in authorize_and_call
    return func(program, project, *args, **kwargs)
  File "/sheepdog/sheepdog/blueprint/routes/views/program/project.py", line 429, in export_entities
    program=program, project=project, **kwargs
  File "/sheepdog/sheepdog/utils/transforms/graph_to_doc.py", line 543, in __init__
    self.get_nodes(ids, with_children)
  File "/sheepdog/sheepdog/utils/transforms/graph_to_doc.py", line 578, in get_nodes
    raise NotFoundError("Unable to find {}".format(', '.join(ids)))
NotFoundError: [404] - Unable to find null
[2018-12-07 20:44:51,446][sheepdog.api][  ERROR] [404] - Unable to find null
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/sheepdog/sheepdog/auth/__init__.py", line 122, in authorize_and_call
    return func(program, project, *args, **kwargs)
  File "/sheepdog/sheepdog/blueprint/routes/views/program/project.py", line 429, in export_entities
    program=program, project=project, **kwargs
  File "/sheepdog/sheepdog/utils/transforms/graph_to_doc.py", line 543, in __init__
    self.get_nodes(ids, with_children)
  File "/sheepdog/sheepdog/utils/transforms/graph_to_doc.py", line 578, in get_nodes
    raise NotFoundError("Unable to find {}".format(', '.join(ids)))
NotFoundError: [404] - Unable to find null

when fixed, un-comment the tests in gen3-qa