nbgrader fetch_feedback creates empty directories

[ykazakov]

When using ngshare exchange in nbgrader, (empty) feedback directories are created also for submissions for which no feedback is provided. This confuses students since nbgrader tells that feedback for these submissions is downloaded.

To reproduce:

1. Install nbgrader with ngshare exchange in jupyterhub.
2. As admin user:
   1. Use ngshare-course-management to create a new course.
   2. Add student john
   3. Create a course nbgrader quickstart cs101
   4. Generate and release assignment bgrader generate_assignment ps1 bgrader release_assignment ps1
3. As user john:
   1. Fetch assignment nbgrader fetch_assignment ps1
   2. Submit assignment (multiple times) nbgrader submit ps1
   3. Fetch feedback nbgrader fetch_feedback
   4. Observe that even though assignments were not marked, feedback is being downloaded:

bash-3.2$ nbgrader fetch_feedback ps1
[FetchFeedbackApp | INFO] Fetching feedback from server
[FetchFeedbackApp | INFO] Successfully decoded feedback for ps1 saved to /private/tmp/2/ps1/feedback/2024-10-25 15:31:59.711872
[FetchFeedbackApp | INFO] Successfully decoded feedback for ps1 saved to /private/tmp/2/ps1/feedback/2024-10-25 15:31:57.543657
[FetchFeedbackApp | INFO] Successfully decoded feedback for ps1 saved to /private/tmp/2/ps1/feedback/2024-10-25 15:31:58.246858
bash-3.2$ tree ps1/feedback/
ps1/feedback/
├── 2024-10-25 15:31:57.543657
├── 2024-10-25 15:31:58.246858
└── 2024-10-25 15:31:59.711872

The fix is coming next...