The content filter has a condition to trip if a header doesn’t exist. However, there is no easy way to catch a header with no content at all.
This rule will trip on a blank subject header (where the header DOES exist, but has no content)
[Rule001]
RuleName=If subject is NOT empty, Skip next rule
Enable=Yes
ThisRuleCondition=All
ProcessQueue=LOCAL
Condition01=SUBJECT|reg exp|AND|^..+$|
Action01=skip rule|"1"
[Rule002]
RuleName=Subject IS empty!
Enable=Yes
ThisRuleCondition=All
ProcessQueue=LOCAL
Condition01=SUBJECT|exists|AND|
Action01=move to bad Msg|
UPDATE 2009/06/17: For some reason this rule was completely broken, both by WordPress inserting smartquotes, and a logic error. My apologies, it has been corrected and tested on MDaemon 10.0.5, it should now work as advertised.