fix: allow empty keyword list for Document

[jennifer-richards]

Without this change, the Document admin won't allow saving a document without at least one keyword in its array. ModelForms involving Document would likely also have this issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.49%. Comparing base (473bbb2) to head (1fa488b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10848      +/-   ##
==========================================
- Coverage   88.49%   88.49%   -0.01%     
==========================================
  Files         332      332              
  Lines       44851    44858       +7     
==========================================
+ Hits        39690    39696       +6     
- Misses       5161     5162       +1     

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