fix: bff handle feedback only criteria

[nsprenkle]

TL;DR - Fix BFF handling of feedback-only rubric criteria.

JIRA: AU-1646

Developer Checklist

• Reviewed the release process
• Translations and JS/SASS compiled
• Bumped version number in openassessment/__init__.py and package.json

Testing Instructions

1. Create an ORA.
2. Edit the ORA to remove all rubric options from one / all of the rubric criteria.
3. Save.
4. Submit a response & progress to end of grading workflow.
5. Verify nothing breaks.

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• I've done a visual code review
• I've tested the new functionality

FYI: @openedx/content-aurora

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f3bd8f1) 94.93% compared to head (29a64fa) 94.92%.

Files	Patch %	Lines
...ent/xblock/ui_mixins/mfe/assessment_serializers.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2154      +/-   ##
==========================================
- Coverage   94.93%   94.92%   -0.01%     
==========================================
  Files         191      191              
  Lines       20570    20571       +1     
  Branches     1864     1865       +1     
==========================================
  Hits        19528    19528              
  Misses        779      779              
- Partials      263      264       +1     

Flag	Coverage Δ
unittests	94.92% <83.33%> (-0.01%)	⬇️

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[mattcarter]

Approved.