Fix auto grading not working on the Grading page for multiple choice questions

[donnapep]

Resolves #6529.

Proposed Changes

I'm unsure of why it started happening now (or maybe it's been an issue for a long time and we just didn't receive reports before now?), but the check to see if user_answers is the same length as correct_answers always failed. This is because, when splitting correct_answers, there's an empty element appended to the array ((2) ['True', '']), so the check would fail and the question would be marked incorrect.

Testing Instructions

1. Add 3 multiple choice questions to a lesson. Ensure that Auto Grade is disabled for the quiz.
2. As a student, take the quiz.
3. In Grading, find the quiz and click Grade quiz.
4. Click the Auto grade button.

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

[github-actions]

WordPress Dependencies Report

The github-action-wordpress-dependencies-report action has detected some script changes between the commit 93ad69a and trunk. Please review and confirm the following are correct before merging.

Script Handle	Added Dependencies	Removed Dependencies	Total Size	Size Diff
js/grading-general.js			2.66 kB	+4 B ( +0.16% 🔼 )

This comment was automatically generated by the github-action-wordpress-dependencies-report action.

[codecov]

Codecov Report

Merging #6884 (93ad69a) into trunk (47552bb) will increase coverage by 0.00%.
The diff coverage is 71.42%.

@@            Coverage Diff            @@
##              trunk    #6884   +/-   ##
=========================================
  Coverage     46.88%   46.88%           
- Complexity    10189    10190    +1     
=========================================
  Files           554      554           
  Lines         36751    36751           
  Branches        381      381           
=========================================
+ Hits          17229    17230    +1     
+ Misses        19216    19215    -1     
  Partials        306      306           

Impacted Files	Coverage Δ
includes/class-sensei-preview-user.php	82.20% <57.14%> (-0.11%)	⬇️
includes/class-sensei-guest-user.php	88.23% <75.00%> (-0.10%)	⬇️
includes/class-sensei-temporary-user.php	86.20% <100.00%> (+0.32%)	⬆️
includes/mailpoet/class-main.php	66.92% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[m1r0]

It's working! 🎉

I've noticed that it still breaks when using a multiple choice question with multiple correct answers but I will try to fix it in a separate PR.