I have constantly been receiving e-mails with the title "No Subject". There is no message in the body of the e-mail and it's size is 28 KB. I also have marked them as spam but they continue to be received.
I don't know, but I think it's safe to say you don't want to know, and you should not open any email that doesn't have a subject and/or a recognizeable sender name/address.
If you haven't done so already I would as a precaution visit www.filehippo.com and download the lastest versions of free antivirus and antiadware/malware applications and run them. There's plenty of them to choose from that are freeware.
Recommend you investigate the mail header source to find out whats happening - though a seasoned individual is able to fake some elements of the mail header data