R1:1cb0d41367b7

Fix inbound mail handling for messages with no HTML body part

Summary:
See D18776. See <https://discourse.phabricator-community.org/t/cant-create-maniphest-tasks-by-email/754/2>.

The change in D18776 to improve handling of non-utf8 HTML parts broke handling of mail with //no// HTML parts. Partly, this is because MimeMailParser has a "traditional" PHP-style API where the return type is an exciting surprise.

Test Plan:
- Sent a text-only message in `Mail.app`.
- Used "Show Raw" to copy it to `mail.