Don't set mail HTML bodies if there's no actual HTML body

Summary:
See https://discourse.phabricator-community.org/t/commit-6011085b0fcd-breaks-sending-certain-email/1571. Some mailers get upset if we setHTMLBody(...) with an empty string.

There's some possible argument they should be more graceful about this, but it's reasonably pretty ambiguous.

Only try to set the HTML body if we actually have a nonempty HTML body.

Test Plan:

• Configured an "smtp" mailer.
• Ran echo hi | ./bin/mail send-test --to someone@somewhere.com --subject test.
• Before: error about empty message body.
• After: no more message body error.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19494