Yes there is a way of doing this. Note that I'm using version 7.5 but I think this would also work in 7.0.2.
Open "Content Handling" in the Policy Option from the policy you want it to apply to. Under "Email Option" and "Empty/Missing Headers", select "When one or more of the following headers are missing or empty". Add "subject" and save the changes. Note that you can add any (RFC 2822) headers you want to check for.