docs: correct 'undefined' to 'null' in callback example

[pirateIV]

Description

The example comment shows undefined but logs null due to sync execution.

Validation

The original example claimed console.log('bar', bar) outputs undefined, but this is incorrect because:

1. bar is explicitly initialized to null (let bar = null).
2. The callback runs synchronously, so bar is still null when logged (the bar = 1 assignment happens later).

Related Issues

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run pnpm format to ensure the code follows the style guide.
• I have run pnpm test to check if all tests are passing.
• I have run pnpm build to check if the website builds without errors.
• I've covered new added functionality with unit tests if necessary.

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[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO	Report
/en	🟠 77	🟢 100	🟢 100	🟢 100	🔗
/en/about	🟢 100	🟢 97	🟢 100	🟠 88	🔗
/en/about/previous-releases	🟢 98	🟢 93	🟢 100	🟠 89	🔗
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/en/blog	🟢 100	🟢 100	🟢 96	🟢 100	🔗

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.04%. Comparing base (dd04024) to head (c709d50).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8037   +/-   ##
=======================================
  Coverage   73.04%   73.04%           
=======================================
  Files          95       95           
  Lines        8324     8324           
  Branches      214      214           
=======================================
  Hits         6080     6080           
  Misses       2243     2243           
  Partials        1        1           

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