fix: add blank=true to SlideSubmission.doc

[jennifer-richards]

Allows admin to save an instance with a null doc reference, which was already permitted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (70bf2ae) to head (8884a20).
Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8688      +/-   ##
==========================================
- Coverage   88.88%   88.80%   -0.08%     
==========================================
  Files         312      314       +2     
  Lines       40891    41243     +352     
==========================================
+ Hits        36345    36627     +282     
- Misses       4546     4616      +70     

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