Fix an issue where "CC"-only email improperly wiped CC addresses

Summary: Ref T920. See https://discourse.phabricator-community.org/t/ccd-emails-not-working-with-sendgrid-since-2019-week-1-update/2294.

Test Plan: Used bin/mail send-test --cc ... without --to, got email.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19970