Jetpack: Stats: Fix csv trailing newline handling

[anomiex]

Proposed changes:

When #39665 switched from incorrectly using str_getcsv( $csv, "\n" ) to explode(), a minor difference between the two was unnoticed: the former does not produce an empty element at the end of the result on a trailing newline, while the latter does.

Add an rtrim on the data to ensure there isn't a trailing newline.

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

p1729086256595799/1729061294.312889-slack-C034JEXD1RD

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

Testing instructions:

• Not sure how to correctly trigger this, but I found going to /wp-admin/admin.php?page=stats&noheader=1&dashboard=1 seems to do it. Note you also need at least some visits to the site. You'll also have to make sure the 'stats_cache' option is cleared.

[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/stats-explode-trailing-newline branch.

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

  bin/jetpack-downloader test jetpack fix/stats-explode-trailing-newline
  

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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