AttributeError when displaying an asset which has only public sensors

[nhoening]

I ran into an error when displaying an asset which has only public sensors (a weather station):

[FLEXMEASURES][2022-10-13 13:06:39,484] ERROR: AttributeError:"'NoneType' object has no attribute 'id'" [occurred at /home/nicolas/workspace/seita/flexmeasures/flexmeasures/data/services/sensors.py(get_sensors):26,URL was: /api/v3_0/assets/10/chart/]
Traceback (most recent call last):
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/flask/app.py", line 1523, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/flask/app.py", line 1509, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/flask_classful.py", line 268, in proxy
    response = view(**request.view_args)
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/flask_classful.py", line 239, in inner
    return fn(*args, **kwargs)
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/webargs/core.py", line 594, in wrapper
    return func(*args, **kwargs)
  File "/home/nicolas/envs/fm-venv/lib/python3.10/site-packages/webargs/core.py", line 594, in wrapper
    return func(*args, **kwargs)
  File "/home/nicolas/workspace/seita/flexmeasures/flexmeasures/auth/decorators.py", line 154, in decorated_view
    return fn(*args, **kwargs)
  File "/home/nicolas/workspace/seita/flexmeasures/flexmeasures/api/v3_0/assets.py", line 287, in get_chart
    return json.dumps(asset.chart(**kwargs))
  File "/home/nicolas/workspace/seita/flexmeasures/flexmeasures/data/models/generic_assets.py", line 296, in chart
    sensors = self.sensors_to_show
  File "/home/nicolas/workspace/seita/flexmeasures/flexmeasures/data/models/generic_assets.py", line 451, in sensors_to_show
    for sensor in get_sensors(
  File "/home/nicolas/workspace/seita/flexmeasures/flexmeasures/data/services/sensors.py", line 26, in get_sensors
    account_ids = [account.id]
AttributeError: 'NoneType' object has no attribute 'id'

This error wasn't happening on main recently, so I believe it was introduced with #500 .

[nhoening]

I guess in get_sensors, we should check for account being None, and then account_ids = [].