fix: coerce multiple choice text elements to String

[cmltaWt0]

Description

"No answer specified" confirmation window is displayed all the time after the second save for dropdown.

Steps to Reproduce:

1. Add new Unit
2. Add new component Problem
3. Choose Dropdown
4. Fill in at least question and answers (for the correct answer, enter the number)
5. Click Save
6. Click on Edit for problem again
7. Click Save

This PR is a possible replacement for #2429

@bradenmacdonald I've create a new PR based on your suggestion here.
If this PR is good for us - I'll close the #2429

[openedx-webhooks]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.90%. Comparing base (42acb53) to head (98b2b11).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2752   +/-   ##
=======================================
  Coverage   94.90%   94.90%           
=======================================
  Files        1235     1235           
  Lines       28260    28261    +1     
  Branches     6433     6433           
=======================================
+ Hits        26819    26820    +1     
  Misses       1370     1370           
  Partials       71       71           

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[bradenmacdonald]

Thanks!