Add return for deprecated Jetpack_Sync_Settings functions

[rebeccahum]

The deprecated functions in Jetpack_Sync_Settings that call the newly re-factored functions expect a returned value but do not return.

Changes proposed in this Pull Request:

• Certain Jetpack_Sync_Settings deprecated functions will now return

Testing instructions:

• Wp shell in
• Call one of the functions where it expects a returned value and see that it returns NULL:

wp> Jetpack_Sync_Settings::is_doing_cron();
=> phar:///root/roles/docker-container-vip-web/usr/local/bin/wp/vendor/wp-cli/shell-command/src/WP_CLI/Shell/REPL.php:52:
NULL

• Make change and call the function again:

wp> Jetpack_Sync_Settings::is_doing_cron();
bool(false)

Proposed changelog entry for your changes:

• Deprecated functions in Jetpack_Sync_Settings with expected return values will return.

[jetpackbot]

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

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 🤖

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[tyxla]

LGTM, thank you for fixing these ❤️