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

src/applications/metamta/storage/PhabricatorMetaMTAMail.php

@@ -917,7 +917,7 @@ final class PhabricatorMetaMTAMail
 
     if ($send_html) {
       $html_body = idx($params, 'html-body');
-
+      if (strlen($html_body)) {
         // NOTE: We just drop the entire HTML body if it won't fit. Safely
         // truncating HTML is hard, and we already have the text body to fall
         // back to.
@@ -926,6 +926,7 @@ final class PhabricatorMetaMTAMail
           $body_limit -= strlen($html_body);
         }
       }
+    }
 
     // Pass the headers to the mailer, then save the state so we can show
     // them in the web UI. If the mail must be encrypted, we remove headers