ImportError when attempting to save an empty Device

[nemesifier]

How to reproduce:

1. go to admin
2. add new device
3. do not fill any information and try to save

Relevant stacktrace:

  File "/var/www/django-netjsonconfig/django_netjsonconfig/models/device.py", line 46, in clean
    self.validate_netjsonconfig_backend(self.backend_instance)
  File "/home/nemesis/.virtualenvs/djnetconfig3/lib/python3.4/site-packages/django/utils/functional.py", line 33, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/var/www/django-netjsonconfig/django_netjsonconfig/models/device.py", line 72, in backend_instance
    return self.get_backend_instance()
  File "/var/www/django-netjsonconfig/django_netjsonconfig/models/device.py", line 79, in get_backend_instance
    backend = self.backend_class
  File "/home/nemesis/.virtualenvs/djnetconfig3/lib/python3.4/site-packages/django/utils/functional.py", line 33, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/var/www/django-netjsonconfig/django_netjsonconfig/models/device.py", line 68, in backend_class
    return import_string(self.backend)
  File "/home/nemesis/.virtualenvs/djnetconfig3/lib/python3.4/site-packages/django/utils/module_loading.py", line 18, in import_string
    six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])
  File "/home/nemesis/.virtualenvs/djnetconfig3/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/home/nemesis/.virtualenvs/djnetconfig3/lib/python3.4/site-packages/django/utils/module_loading.py", line 15, in import_string
    module_path, class_name = dotted_path.rsplit('.', 1)
ImportError:  doesn't look like a module path