Comments Connector: Avoid a PHP Warning on pingbacks.

[dd32]

A PHP Warning is triggered on sites that require comment registrations, during pingback creations.

Eg.

E_WARNING: Attempt to read property "display_name" on bool in /wp-content/plugins/stream/connectors/class-connector-comments.php:200
Source: POST https://example.org/xmlrpc.php
UA: The Incutio XML-RPC PHP Library -- WordPress/6.7.2
Filter stack: wp_insert_comment

This is because WP_Comment->user_id will always exist, although it may be empty.

This PR does two things:

• Check that the user_id is empty (ie. not 0)
• Copies over the existing logic for when a user can't be found via email, in the event the user object can't be found.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Avoid triggering PHP Warnings under PHP 8.x

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.