Fix gap fill question to accept "0" as the right answer

[renatho]

Issue reported in p1699118258790299-slack-CAXUNK9MW

Proposed Changes

• It fixes an issue with the gap fill question, which wasn't accepting "0" as the right answer.

Testing Instructions

1. Create a course with a quiz.
2. Add a gap fill question.
3. Add "0" as the right answer.
4. Refresh the editor, and make sure the answer continue there.
5. Answer the quiz as a student and make sure it works properly.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7273 (300d85f) into trunk (61fefe5) will increase coverage by 0.01%.
Report is 11 commits behind head on trunk.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##              trunk    #7273      +/-   ##
============================================
+ Coverage     50.59%   50.60%   +0.01%     
- Complexity    10975    10976       +1     
============================================
  Files           608      608              
  Lines         46286    46286              
  Branches        402      402              
============================================
+ Hits          23417    23422       +5     
+ Misses        22542    22537       -5     
  Partials        327      327              

Files	Coverage Δ
...des/cli/class-sensei-validate-progress-command.php	0.00% <ø> (ø)
...igration/validations/class-progress-validation.php	85.34% <ø> (ø)
...l/migration/validations/class-validation-error.php	100.00% <ø> (ø)
...i/class-sensei-rest-api-question-helpers-trait.php	92.78% <100.00%> (+0.80%)	⬆️

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[m1r0]

Works as described. 💯

I would love to see some tests, but it looks like a tricky thing to do on that piece of code. I will leave that decision up to you. Approving either way.

[renatho]

I would love to see some tests, but it looks like a tricky thing to do on that piece of code. I will leave that decision up to you. Approving either way.

Good point, @m1r0! Previously I had searched for tests of that trait class, and I skipped thinking that it would be complex. But after your comment, I checked a little more, and I found the tests of the class using that, so I added the tests there 300d85f. Thank you for the suggestion!

[m1r0]

Nice! That was fast! ⚡