Fix 500 error that occurred when no fields changed

[U-238]

Using Django 6.0 and django-reversion 6.1.0, I was getting a 500 error when saving a model where no fields had changed. This PR fixes this bug.

[etianen]

Sorry, I just noticed this pr 😮

Can you provide a traceback for the error this fixes? It would be good to verify that this fixes the problem before merging it blind 😄

[U-238]

Hi @etianen, no problem. I've just confirmed the bug still exists in django-reversion 6.3.0, running on Python 3.12 with Django 6.0.6. Here is the stack trace:

Traceback (most recent call last):
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 198, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/contrib/admin/options.py", line 711, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/utils/decorators.py", line 192, in _view_wrapper
    result = _process_exception(request, e)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/utils/decorators.py", line 190, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/views/decorators/cache.py", line 80, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/contrib/admin/sites.py", line 247, in inner
    return view(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/reversion/admin.py", line 180, in change_view
    return super().change_view(request, object_id, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/contrib/admin/options.py", line 1946, in change_view
    return self.changeform_view(request, object_id, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/utils/decorators.py", line 48, in _wrapper
    return bound_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/utils/decorators.py", line 192, in _view_wrapper
    result = _process_exception(request, e)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/utils/decorators.py", line 190, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/contrib/admin/options.py", line 1802, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/django/contrib/admin/options.py", line 1862, in _changeform_view
    self.log_change(request, new_object, change_message)
  File "/home/me/code/sytycb/.venv/lib/python3.12/site-packages/reversion/admin.py", line 97, in log_change
    set_comment(entry.get_change_message())
                ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get_change_message'

Then I tried applying the same fix as in this PR, and the bug no longer occurred.