Stop LLMFreeTextQuestions from generating choices-based content warnings

[sjd210]

isaacLLMFreeTextQuestions do not require a choices field, since they use a markScheme as its equivalent.
We should therefore not be giving a warning when the choices field is missing.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 40.31%. Comparing base (c5d1f1c) to head (f229d8f).

Files with missing lines	Patch %	Lines
...ava/uk/ac/cam/cl/dtg/segue/etl/ContentIndexer.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #791      +/-   ##
==========================================
- Coverage   40.31%   40.31%   -0.01%     
==========================================
  Files         547      547              
  Lines       23773    23773              
  Branches     2893     2893              
==========================================
- Hits         9584     9583       -1     
  Misses      13288    13288              
- Partials      901      902       +1     

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[jsharkey13]

This feels like an acceptable stop-gap. But I would love to review the inheritance hierarchy here, and try to sort out the mess of IsaacQuestionBase -> ChoiceQuestion being back to front (particularly since we're removing the segue/isaac divide over time).

If IsaacLLMQuestion didn't extend ChoiceQuestion, it logically wouldn't be expected to have choices; but right now, things would break if it did not 😣

[sjd210]

@jsharkey13 I've also realised (to add to the mess) that we absolutely ARE expecting these questions to have the markScheme field, and that field must not be an empty list - so that should probably be its own content warning in lieu of this one.