Fix Image Compare block in AMP when width/height are not available

[westonruter]

Fixes #16093
See #14395

All elements in AMP need to have dimensions defined up-front so that the page will have static layout to avoid CLS (Cumulative Layout Shift). When no width or height is available for the Image Compare block, instead of omitting the width/height for the amp-image-slider component, the correct AMP fallback is to provide fallback dimensions. Since we don't know what the image dimension is, providing an aspect ratio of 1:1 seems to make sense.

Changes proposed in this Pull Request:

• Ensure fallback dimensions are provided to Image Compare block when image size is not available

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Add an Image Compare block to a post and view that post on an AMP page.

I'm not sure entirely why the width or height weren't available when calling render_amp(), so the only way I know to test this is to forcibly unset them when calling the function:

unset( $img_before['width'], $img_before['height'] );

Proposed changelog entry for your changes:

• Fix AMP implementation of Image Compare block

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17253

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 667e3e1

[jeherve]

This tests well as well. 🚢

[jeherve]

r214169-wpcom