Image Block: Don't render if there is no URL set

[t-hamano]

Fix #44639

Note
This PR probably depends on #43178

What?

This PR disables rendering on the front end when no URL is set in the image block.

Why?

When the image block doesn't have a URL, the front end outputs HTML like this:

<figure class="wp-block-image"><img decoding="async" alt=""></figure>

According to MDN, the img element must have src attribute, so I believe this is incorrect markup.

The src attribute is required, and contains the path to the image you want to embed.

How?

This PR uses the new HTML Tag Processor API introduced in #42485 and changed to output nothing if the src attribute is empty. This rule is similar to the one that outputs nothing if the image is empty in the following block:

• Post Featured Image
• Site Logo
• Social Link

Also, a refactoring using this API in the core block in general is proposed by #43178. So this PR may need to be updated after #43178 is merged to prevent conflicts.

Testing Instructions

1. After inserting the image block, save the article without specifying an image.
2. Confirm that nothing is output to the front end.

[t-hamano]

Thank you for the review, @Mamaduka! I rebased with the latest trunk and adjusted the code.

[github-actions]

Flaky tests detected in a7ea505.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4486800784
📝 Reported issues:

• [Flaky Test] allows shift tabbing to the block toolbar from the first block #49025 in /test/e2e/specs/site-editor/writing-flow.spec.js
• [Flaky Test] allows tabbing to the inspector from the last block #48504 in /test/e2e/specs/site-editor/writing-flow.spec.js
• [Flaky Test] can create template parts via the block placeholder start blank option #49016 in /test/e2e/specs/site-editor/template-part.spec.js
• [Flaky Test] doesn't throw a block error when clicking on a link #47747 in /test/e2e/specs/site-editor/template-part.spec.js

[Mamaduka]

Thank you, @t-hamano! The changes work as expected.

[t-hamano]

@Mamaduka
Sorry, I thought I merged as usual, but the latest commit message "Rebase with trunk" was recorded in trunk 🙏

[Mamaduka]

No worries 😉