Crash in grader statistics computation

```
Traceback (most recent call last):
File "/home/andreas/relate/.venv/lib/python3.12/site-packages/django/core/handlers/exception.py",
line 55, in inner
response = get_response(request)
^^^^^^^^^^^^^^^^^^^^^
File "/home/andreas/relate/.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/andreas/relate/course/utils.py", line 821, in wrapper
response = f(pctx, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/andreas/relate/course/grading.py", line 490, in show_grader_statistics
pages = sorted(pages)
^^^^^^^^^^^^^

Exception Type: TypeError at /course/cs450-f24/grading/statistics/hw-1/
Exception Value: '<' not supported between instances of 'int' and 'NoneType'
Raised during: course.grading.show_grader_statistics
```

Start at course/grading.py:490 in show_grader_statistics and reproduce the grading statistics URL shown in the traceback. Trace how pages is populated and determine the expected handling for None; done means the statistics page no longer raises this TypeError, with regression coverage for this path if available.

Written by the indexing model from the issue text.