Janitorial: Handle deprecation notices found in wild

[tbradsha]

During the deploy of Jetpack 14.4-a.3 I noticed many preexisting deprecation notices. This PR tackles the most prevalent ones.

Proposed changes:

Below is the count of errors in the last 96 hours, the file in question, and the fix:

Count	File	Fix
50000	/jetpack_vendor/automattic/jetpack-videopress/src/class-divi.php	declare class property
12000	/modules/carousel/jetpack-carousel.php	do a string type check
6000	/modules/widgets/contact-info.php	null coalesce to string
7500	/modules/markdown/easy-markdown.php	null coalesce to string
50	/class.jetpack-post-images.php	null coalesce to string

In many cases, the variable was always assumed to be a string, so null coalesce was the answer. For jetpack-carousel.php it was passed as a variable, so I both set the default param value to '' and did a string check.

There are still many other deprecation notices, but the rest had very few occurrences so I'm deferring them to a later fix.

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

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

Testing instructions:

Do the changes make sense?

[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/jetpack/handle_deprecation_notices_in_wild_found_during_14.4-a.3_deploy branch.

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

  bin/jetpack-downloader test jetpack fix/jetpack/handle_deprecation_notices_in_wild_found_during_14.4-a.3_deploy
  

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!

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[github-actions]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/modules/carousel/jetpack-carousel.php	54/615 (8.78%)	0.13%	1 ❤️‍🩹

Full summary · PHP report · JS report