fix: add null check and use getLocFromIndex in require-alt-text

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

The original intention of this PR was to refactor the code to use getLocFromIndex, but I found a small bug while working on it, so I fixed it as well.

Which language are you using?

CommonMark and GFM.

What did you do?

<img src="image.png" aria-hidden alt="alt">

When I run the code above, the rule crashes for the following reason: a missing null check.

(FYI, the code above is recognized as valid syntax even without true or false following it.)

What did you expect to happen?

No crash would happen.

Link to minimal reproducible Example

'<img src="image.png" aria-hidden alt="alt">'

Adding the above code to the valid section of require-alt-text.test.js would help identify the problem.

What changes did you make? (Give an overview)

The original intention of this PR was to refactor the code to use getLocFromIndex, but I found a small bug while working on it, so I fixed it as well.

This PR originated from #536. While working on that, I found the scope had become too large, so I split this refactoring into a separate PR.

In this PR, I've used the native getLocFromIndex method added in #376 instead of the custom findOffsets util.

Notable changes:

• Replace the findOffsets util with the native getLocFromIndex.
• Use ESQuery selectors to reduce duplicated logic.
• Fix a bug.

Related Issues

Ref: #376, #536

Is there anything you'd like reviewers to focus on?

N/A

[lumirlumir]

This is the only place where the bug fix is needed (it's a one-line change).

Other changes are related to code refactoring.

I could use optional chaining, but I used this check to match the implementation on line 97.

[lumirlumir]

To apply the feedback from #541, I'll mark this PR as a draft shortly.

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All clear.

[mdjermanovic]

LGTM, thanks!