[bug] Device Firmware: submitting empty image triggers uncaught exception

[nemesifier]

Describe the bug

Traceback (most recent call last):
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/core/handlers/exception.py", line 42, in inner
    response = await get_response(request)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/core/handlers/base.py", line 253, in _get_response_async
    response = await wrapped_callback(
  File "/usr/lib/python3.10/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/contrib/admin/options.py", line 719, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/utils/decorators.py", line 192, in _view_wrapper
    result = _process_exception(request, e)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/utils/decorators.py", line 190, in _view_wrapper
    response = view_func(request, *args, **kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/views/decorators/cache.py", line 80, in _view_wrapper
    response = view_func(request, *args, **kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/contrib/admin/sites.py", line 246, in inner
    return view(request, *args, **kwargs)
  File "/Code/openwisp/openwisp-controller/openwisp_controller/config/admin.py", line 164, in change_view
    return super().change_view(request, object_id, form_url, extra_context)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/reversion/admin.py", line 162, in change_view
    return super().change_view(request, object_id, form_url, extra_context)
  File "/Code/openwisp/openwisp-utils/openwisp_utils/admin.py", line 126, in change_view
    return super().change_view(
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/contrib/admin/options.py", line 1987, in change_view
    return self.changeform_view(request, object_id, form_url, extra_context)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/utils/decorators.py", line 48, in _wrapper
    return bound_method(*args, **kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/utils/decorators.py", line 192, in _view_wrapper
    result = _process_exception(request, e)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/utils/decorators.py", line 190, in _view_wrapper
    response = view_func(request, *args, **kwargs)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/contrib/admin/options.py", line 1843, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/contrib/admin/options.py", line 1893, in _changeform_view
    if all_valid(formsets) and form_validated:
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/formsets.py", line 584, in all_valid
    return all([formset.is_valid() for formset in formsets])
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/formsets.py", line 584, in <listcomp>
    return all([formset.is_valid() for formset in formsets])
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/formsets.py", line 384, in is_valid
    self.errors
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/formsets.py", line 366, in errors
    self.full_clean()
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/formsets.py", line 429, in full_clean
    form_errors = form.errors
  File "/.virtualenvs/openwisp2/lib/python3.10/site-packages/django/forms/forms.py", line 201, in errors
    self.full_clean()
  File "/Code/openwisp/openwisp-firmware-upgrader/openwisp_firmware_upgrader/admin.py", line 359, in full_clean
    device=self.cleaned_data["device"],

Exception Type: KeyError at /admin/config/device/5131f85b-5604-46be-a0da-b45ded388a08/change/
Exception Value: 'device'

Steps To Reproduce
Steps to reproduce the behavior:

1. Go to device detail page in admin
2. Go to "Firmware" tab
3. Click on "Add another firmware"
4. Do not set the image field and submit

Expected behavior

A ValidationError should be raised instead, which will be shown in the admin UI.

System Informatioon:

Not relevant.