REST API: Fix user connection data without owner

[fgiannar]

This PR fixes a PHP notice when trying to fetch user connection data while a connection owner does not exist (user-less).

Changes proposed in this Pull Request:

• Updates the get_user_connection_data method in Jetpack_Core_Json_Api_Endpoints to properly handle cases where the connection owner does not exist.
• Adds a corresponding unit test

Jetpack product discussion

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

No

Testing instructions:

Manual

Before applying this fix:

• Start with a fresh Jetpack site
• Enable user-less connection mode by setting JETPACK_NO_USER_TEST_MODE ( in wp-config.php or via Settings -> Jetpack Constants)
• Set up Jetpack in user-less mode (aka skip user authentication)
• Go to Jetpack Dashboard and notice the following notice when fetching: jetpack/v4/connection/data:
  The notice will be visible either in debug.log or in your browser console, Network tab -> XHR -> GET wp-json/jetpack/v4/connection/data:

PHP Notice: Trying to get property 'display_name' of non-object in /usr/local/src/jetpack-monorepo/projects/plugins/jetpack/_inc/lib/class.core-rest-api-endpoints.php on line 1598

• Switch to this branch and confirm that the Notice is gone.

Automated

phpunit --filter=test_get_user_connection_data_without_master_user

Proposed changelog entry for your changes:

• REST API: fix PHP notice when fetching user connection data without a connection owner.

[jetpackbot]

Scheduled Jetpack release: February 2, 2021.
Scheduled code freeze: January 25, 2021

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.

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[sergeymitr]

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