It's not possible to make a rule with an empty field in Mail.app on OS X apparently. The solution I found is to list all vowels as necessarily missing in order to process the message (in this example case, move it to the trash).
Note: this does not deal with images or attachments of any type as the sole content of the message body however. An attachment does count as 'content' in the message body. The rule(s) and criteria would have to be quite extensive to deal with attachments in addition to non-character message content.