Add null check for summary.NextElementSibling in global.js

[vitorascorrea]

PR Summary:

This PR adds a check in global.js that prevents trying to access a <summary> element sibling if it doesn't exist.

Why are these changes introduced?

Fixes #3452

What approach did you take?

Use optional chaining on the nextElementSibling query.

Visual impact on existing themes

Merchants that have a <summary> element that matches the default global.js selector but don't have a sibling won't cause the page's javascript to thrown an exception.

Testing steps/scenarios

Customizing the theme's product page by adding an html snippet like the one below shouldn't break option/variant selection:

<div id="Details-001">
  <details>
    <summary>This is the only child</summary>
    <!-- There is no nextElementSibling for this summary -->
  </details>
</div>

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility