fix: leave new dropdown answers empty instead of "undefined"

[ihor-romaniuk]

Description

Adding a dropdown problem in Studio used to fill all three answers with the word undefined. The fields look filled in, their "Enter an answer" placeholder never shows, and an author has to clear each one before typing. Every other problem type starts with empty answers.

New dropdown answers now start empty, like the rest.

Implementation notes

• The blank dropdown template writes <option correct="True"></option>, so the parsed element has no #text. OLXParser.parseMultipleChoiceAnswers ran that through String(), which turns undefined into the string "undefined".
• Single select and multi-select never showed it because they are in RichTextProblems: their titles are rebuilt from the preserved rich text on the very next line. Dropdown answers are plain text, so the string survived.
• ?? '' rather than a falsy check, so an option whose text is 0 or false still parses to that value.
• Both branches of the parser are fixed — the one for several options and the one for a single option.

Screenshot/Video

Before	After

Testing

1. In Studio, add a new Problem component to a unit and pick Dropdown.
2. Expected: the three answer fields are empty and show the "Enter an answer" placeholder. Before this change each held the word undefined.
3. Type an answer into the first field and save, then reopen the editor.
4. Expected: the answer is what you typed.
5. Repeat step 1 for Single select, Multi-select, Numerical input and Text input.
6. Expected: unchanged — all of them already started empty.
7. Open an existing dropdown problem that has answers.
8. Expected: its answers are unchanged.

Verified locally

Check	Result
New dropdown problem in the running Studio	three answer fields empty with the placeholder visible; before the fix each held undefined
Parser on the blank templates	dropdown now yields ["", "", ""], matching single select; before it yielded ["undefined", "undefined", "undefined"]
Regression test in OLXParser.test.js	the blank dropdown template parses to three empty titles
Red/green	restoring the old String() call fails exactly that test
jest src/editors/containers/ProblemEditor	44 suites, 326 tests
tsc, dprint, oxlint	clean, including the stricter upstream rule set
Not verified	Only the five basic problem types were checked by hand; advanced types open in the raw OLX editor and do not go through this parser path.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.92%. Comparing base (16e19f1) to head (1c203ff).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3216    +/-   ##
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  Coverage   95.92%   95.92%            
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  Files        1397     1397            
  Lines       33581    33581            
  Branches     7947     7678   -269     
========================================
  Hits        32214    32214            
- Misses       1308     1323    +15     
+ Partials       59       44    -15     

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