Image Block: explicitly check for false from strpos()

[ramonjd]

Description

Following on from the conversation in #35975 (review)

This PR explicitly checks for a return value of false from strpos() just in case the check returns 0, which would also be falsey and satisfy the if-statement's condition.

If data-id were at the leading position in the string though, we'd have other problems :)

Props to @mcsf for catching it.

Testing Instructions

Insert a gallery and a single image into a post. Publish the page and head over to your site.

Make sure that the published gallery images include the data-id attribute on the <img /> tags. The single image will not have the data-id attribute.

Screenshots

Types of changes

Code qual.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[aaronrobertshaw]

LGTM 👍

✅ Gallery images get the data-id attribute
✅ Single images do not get the data-id attribute

[mcsf]

Thanks!