autoloader: Avoid a deprecation notice in Autoloader_Locator::find_latest_autoloader()

[anomiex]

Proposed changes:

If one of the cached plugin paths has gone missing, get_autoloader_version() will return null when trying to check it. Avoid passing that null to is_version_update_required().

This sort of thing happens weekly during Atomic upgrades, because there they have symlinks wp-content/plugins/jetpack →
/wordpress/plugins/jetpack/latest → /wordpress/plugins/jetpack/<ver> and the update process deletes the old <ver> at the same time as creating the new and updating the latest symlink.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1720462998876079/1720457892.196259-slack-C01U2KGS2PQ

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

No

Testing instructions:

Without this PR, this process should produce deprecation warnings about passing null to substr and version_compare. With it, it should no longer do so.

• Move wp-content/plugins/jetpack to wp-content/plugins/jetpack-xxx, then symlink wp-content/plugins/jetpack to that. Load a page.
• Move wp-content/plugins/jetpack-xxx to wp-content/plugins/jetpack-yyy, then symlink wp-content/plugins/jetpack to that. Load a page.

(if for some reason the PR doesn't seem to fix it, ensure that the PR's version of the autoloader is active rather than a trunk version from some other plugin)

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/autoloader-atomic-deprecated-notice branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/autoloader-atomic-deprecated-notice
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
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• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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