StatusItem.datetime_since can be None

[Curtis Hovey]

As seen at
    http://reports.vapour.ws/releases/issue/5841b611749a5634f20d6612

Older Jujus do not have since, or an explanation for the status at all.

Traceback (most recent call last):
  File "/mnt/jenkins/juju-ci-tools/utility.py", line 553, in logged_exception
    yield
  File "/mnt/jenkins/juju-ci-tools/deploy_stack.py", line 871, in runtime_context
    yield
  File "/mnt/jenkins/juju-ci-tools/deploy_stack.py", line 978, in booted_context
    yield machines
  File "/mnt/jenkins/juju-ci-tools/deploy_stack.py", line 1072, in _deploy_job
    deploy_dummy_stack(client, charm_series)
  File "/mnt/jenkins/juju-ci-tools/deploy_stack.py", line 113, in deploy_dummy_stack
    client.wait_for_started(3600)
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 2108, in wait_for_started
    timeout=timeout, start=start)
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 2091, in _wait_for_status
    status.raise_highest_error(ignore_recoverable=True)
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 948, in raise_highest_error
    errors = self.check_for_errors(ignore_recoverable)
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 944, in check_for_errors
    key=lambda item: item.priority())
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 936, in iter_errors
    error = sub_status.to_exception()
  File "/mnt/jenkins/juju-ci-tools/jujupy.py", line 752, in to_exception
    time_since = datetime.now(tz.gettz('UTC')) - self.datetime_since
TypeError: unsupported operand type(s) for -: 'datetime.datetime' and 'NoneType'