Fix php warnings subscription widget

[simison]

Fixes some PHP warnings I noticed in logs, blurred customer details:

Warning: Undefined array key "title_following" in public_html/wp-content/mu-plugins/jetpack-plugin/moon/modules/subscriptions/views.php on line 726 [example.wordpress.com/wp-admin/customize.php?url=https%3A%2F%2Fexample.wordpress.com%2Fexample%2F] [wp-includes/class-wp-widget.php:553 form(), wp-admin/includes/widgets.php:276 form_callback(), wp-includes/class-wp-customize-widgets.php:1148 wp_widget_control(), wp-includes/class-wp-customize-widgets.php:1096 get_widget_control(), wp-includes/class-wp-customize-widgets.php:714 get_available_widgets(), wp-includes/class-wp-hook.php:324 enqueue_scripts(), wp-includes/class-wp-hook.php:348 apply_filters(''), wp-includes/plugin.php:517 do_action(Array), wp-admin/customize.php:127 do_action('customize_controls_enqueue_scripts')]

Proposed changes:

• PHP warning fixes

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:

• Code review should be enough
• Alternatively test Customizer and subscription widget with classic theme

[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/php-warnings-subscription-widget branch.

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

  bin/jetpack-downloader test jetpack fix/php-warnings-subscription-widget
  

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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[monsieur-z]

I haven't tested the PR but the code changes look straightforward.