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.txt, verifying that the raw message contains ONLY a text body.
• Ran cat mail.txt | ./scripts/mail/mail_handler.php --trace --process-duplicates.
  • Before patch: error about bad idx() on a non-array.
  • After patch: clean mail processing.
• Did the same with a message with both HTML and text bodies to make sure I didn't break anything.

Ideally we'd probably get test coverage on this, but it's been touched roughly once a year since 2013 so it'll probably hold.

Reviewers: amckinley, alexmv

Reviewed By: amckinley, alexmv

Differential Revision: https://secure.phabricator.com/D18778