You can do this by switching your thinking around a little to make Outlook look to see if the subject line is NOT blank.
Create a template for the message you want to send in reply. Then create a rule which will use the exception: "subject line contains specific words". You will add single letters into the exception criteria. Vowels work best because most subject lines are going to have them in it.
In essence, this rule will look to see if the subject line meets the criteria. If not (blank), it will reply with the template you choose.
Here are detailed instructions: Rule to AutoReply all correspondence without subject.