General: fix PHP notices that may pop up with PHP 7.4.

[jeherve]

Changes proposed in this Pull Request:

• PHP 7.4 now throws a notice if you try to access a resource as an array, when it is not an array, a string, or an object.
  https://wiki.php.net/rfc/notice-for-non-valid-array-container

PHP Notice:  Trying to access array offset on value of type bool in /var/www/html/wp-content/plugins/jetpack/_inc/lib/admin-pages/class.jetpack-react-page.php on line 349
PHP Notice:  Trying to access array offset on value of type bool in /var/www/html/wp-content/plugins/jetpack/src/class-tracking.php on line 95

Testing instructions:

• Update your Docker container to PHP 7.4 as per Update docker to use PHP 7.4 #13773
• Connect your site to WordPress.com, and visit a few Jetpack dashboard pages.
• You should not see any notices in your logs.

Proposed changelog entry for your changes:

• General: fix PHP warnings that may appear on sites running PHP 7.4.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeherve! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D36380-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: January 14, 2020.
Scheduled code freeze: January 7, 2020

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