Multiple undefined indexes during csv import

Not all of the columns are necessary, so these are often not defined: https://github.com/Automattic/Co-Authors-Plus/blob/master/php/class-wp-cli.php#L822-L828

Same for `$author['user_email']`, `$author['first_name']`, `$author['last_name']`, etc.

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On another note, user_email is required, otherwise the checks after this fail because it thinks the current user being imported is the same as another user without an email: https://github.com/Automattic/Co-Authors-Plus/blob/master/php/class-wp-cli.php#L851. Should instead make sure it's not empty first.

Inspect php/class-wp-cli.php around lines 822-828 and 851, starting with the CSV import handling and its author field checks. Confirm that optional author fields do not produce undefined-index notices and that empty user_email values do not trigger the duplicate-user check; verify the import behavior with the relevant CSV import tests or command.

Written by the indexing model from the issue text.