Don't add "To/Cc" to HTML mail bodies if there's no HTML mail body

Summary: Ref T6576. This avoids generating almost-empty HTML mail bodies for mail which incorrectly has no HTML body.

Test Plan: Generated some mail locally; the specific hook case is a pain for me to hit right now. Will push and dig in if that doesn't fix it.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6576

Differential Revision: https://secure.phabricator.com/D10863