Fixing a test for None on both variables in body and html

If body variable didn't exist and html did, pyramid_mailer was assuming both were empty, note that I only discovered this when moving from message.send() to message.send_to_queue(), but nevertheless this fix does the same thing as before, but this actually is a true test on both variables being empty.
This can't be right:  we want at least one of 'body' or 'html' to be set (both would be fine).  The test as you changed it will barf if the message does have 'body', but doesn't have 'html' (not an error condition).