Fix a PHP warning when iterating over menu items in the masterbar module

[ice9js]

Proposed changes:

This aims to fix an occasional PHP warning that pops up when iterating over menu items - some of which are null, causing issues with the foreach loop.

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?
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Jetpack product discussion

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

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Testing instructions:

TBA

[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/admin-menu-php-warning branch.

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

  bin/jetpack-downloader test jetpack fix/admin-menu-php-warning
  

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• 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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