R1:a7c681b54997

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…