This is a known bug but I haven't looked for the cause yet.  Further updates will be to support #101979

Sendmail 8.13.1, Spamassassin 2.63, Spamass-milter 0.2.0, Solaris
If an email has an empty "subject:" , and is spam, Spamassassin will add [SPAM] to the original Subject, but the milter will add a second empty "Subject:" line to the email , which will cause the mailer (netscape in my case) to show an empty Subject in the email list.

Any solution ? easly re-producable, try this :
paste this into a tcp session to port 25 of your email server, in the data part, and let the milter go over it
Subject:

viagra 
.
quit

This should be tagged as spam, and if you look at the email header, you will see two subjects.