[WC Analytics] Fix fatal error when cart is null

[puntope]

Fixes https://a8c.slack.com/archives/C01U2KGS2PQ/p1735038171598949?thread_ts=1734981348.766769&cid=C01U2KGS2PQ

Proposed changes:

• Fixes a fatal when WC()->cart returns null when tracking the session_statted event when sending headers

PHP Fatal error:  Uncaught Error: Call to a member function get_subtotal() on null in /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-woo-analytics-trait.php:598
Stack trace:
#0 /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-woo-analytics-trait.php(275): Automattic\Woocommerce_Analytics\Universal->get_cart_subtotal()
#1 /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-woo-analytics-trait.php(394): Automattic\Woocommerce_Analytics\Universal->get_common_properties()
#2 /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-woo-analytics-trait.php(314): Automattic\Woocommerce_Analytics\Universal->process_event_properties('woocommerceanal...', Array, NULL)
#3 /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-universal.php(69): Automattic\Woocommerce_Analytics\Universal->record_event('woocommerceanal...')
#4 /wordpress/core/6.7.1/wp-includes/class-wp-hook.php(324): Automattic\Woocommerce_Analytics\Universal->initialize_woocommerceanalytics_session(Object(WP))
#5 /wordpress/core/6.7.1/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array)
#6 /wordpress/core/6.7.1/wp-includes/plugin.php(565): WP_Hook->do_action(Array)
#7 /wordpress/core/6.7.1/wp-includes/class-wp.php(590): do_action_ref_array('send_headers', Array)
#8 /wordpress/core/6.7.1/wp-includes/class-wp.php(821): WP->send_headers()
#9 /wordpress/core/6.7.1/wp-includes/functions.php(1336): WP->main('')
#10 /wordpress/core/6.7.1/wp-blog-header.php(16): wp()
#11 /wordpress/core/6.7.1/index.php(17): require('/wordpress/core...')
#12 {main}
  thrown in /wordpress/plugins/jetpack/14.2-a.5/jetpack_vendor/automattic/woocommerce-analytics/src/class-woo-analytics-trait.php on line 598

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:

I wasn't able to reproduce the error. BUt I forced it to check that is not happening when cart is null by adding

WC()->cart = null; in https://github.com/Automattic/jetpack/blob/trunk/projects/packages/woocommerce-analytics/src/class-woo-analytics-trait.php#L259

Then visit the shop or any page and check that there is no fatals.

[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/analytics-error-cart-null branch.

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

  bin/jetpack-downloader test jetpack fix/analytics-error-cart-null
  

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]

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