Identity Engine lookup_group() Method Raises Exceptions When No Groups Found

[jeffwecan]

Vault Version: 0.11.1
hvac Version: 0.7.0

  File "/usr/local/lib/python2.7/dist-packages/hvac/api/secrets_engines/identity.py", line 1031, in lookup_group
    return response.json()
  File "/usr/local/lib/python2.7/dist-packages/requests/models.py", line 897, in json
    return complexjson.loads(self.text, **kwargs)
  File "/usr/lib/python2.7/json/__init__.py", line 339, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 364, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 382, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

Would probably make more sense to return a None in these cases?