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 fix does not do what you say it does.

    if self.body

Simply checks that `self.body` is `Truthy`. 

    if self.body and self.html == None:

Checks that `self.body` is `Truthy` and that `self.html` is None.

It does NOT check that `self.body` and `self.html` are both empty.

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The previous code was checking that `self.body` and `self.html` are BOTH NOT `Truthy`, and THEN raising an error message.